Aziz Sancar


Aziz Sancar


“On 11 March 1890, a five-hour banquet for hundreds of invited guests was held in the festive chamber of the Berlin City Hall. A festival of a magnificence perhaps unparalleled in the history of science … The vast chandeliered room was decorated with palm trees and laurel leaves, and one end was dominated by a five-metre-high oil painting of Bismarck and other European statesmen carving up the Turkish empire at the Congress of Berlin.”

I first read this paragraph in 2004, in John Buckingham’s excellent book about the history of chemistry, Chasing the Molecule [1]. The banquet was in honor of Kekulé, who is the main formulator of the theory of chemical structure (the theory that all molecules have definitive 3-dimensional structures) and whose discovery of the hexagonal molecular structure of benzene in 1865 was a major breakthrough in both pure and applied chemistry. I was impressed by the central theme of the Benzolfest and the celebration of chemistry, and of science in general, described in the Introduction, but also struck by the cavalier attitude of the Europeans of that period – and apparently of the author – about “carving up the Turkish Empire.” These two subjects, science and the Turkish Nation (Ottoman Empire and Republic of Turkey), not necessarily in that order, have dominated my thinking for as long as I can remember. I grew up as, and still am, a Turkish patriot and from the age of about 10 I was also an aspiring, and later practicing, scientist.

The early years

I was born on September 8, 1946, in a small town named Savur in the Mardin Province of southeastern Turkey, the seventh of eight children of Abdulgani and Meryem Sancar. I also had two half-brothers. Father was a farmer, and Mother took care of the children and the house. By the standards of the day we were a lower middle-class family. We always had enough to eat, but shoes were luxuries, and until the seventh grade we wore them only when we went to school. Much of my early youth was spent in the valley below our house where, alongside my brothers and father, I tended the fruit and nut trees and the vegetable garden that provided our family nourishment and income. We also had a few farm animals that provided milk and meat for our family throughout the year. My most pleasant memories from childhood are the flowering of the almond and plum trees in our orchard in the spring. In those early years, I began to learn about Islam and was convinced that Paradise must look like our orchard when the almond trees were in full bloom.

Meryem Sancar
Figure 1. Meryem Sancar pregnant with Aziz in 1946 in Savur, Turkey. The little girl in the background is the 4-year old Seyran, Aziz’ youngest sister.

Overall, I did not like farm work. The terraces in the vegetable garden were held in place by stone walls constructed without mortar and required constant maintenance by me and my brothers. Walnut harvesting was hard work, and as one of the younger children, I had to climb very high into the trees to make sure all the walnuts fell. But the worst was herding baby goats, because they could run faster than any 5–7 year old boy. My younger brother and I were in charge of herding them and spent many terrified hours trying to find the runaways before Father noticed they were gone.

Aziz in 3rd grade.
Figure 2. Aziz in 3rd grade with big brother Kenan and sister-in-law Nezihe (1955).

Our large extended family was an important part of my early childhood. Uncles, aunts, and many cousins lived in Savur, and there were often other relatives visiting from towns farther away. Visits with my Uncle Sevket and his family in Mardin City were another high point. Mardin is known for its beautiful architecture which dates primarily from 1100 A.D.–1300 A.D. Sleeping in large beds on the rooftop of Uncle’s house was always a treat. As I fell asleep, I would watch on the horizon the lights of two nearby Syrian towns, and in the morning I would wake to the call to prayer from the historic Sehidiye Mosque about 200 meters from our house.

Early influences

The three most important influences in my early education, in addition to Mustafa Kemal Ataturk, were my mother Meryem, my father Abdulgani, and Kenan, my oldest brother. Beginning in 1911, and until the end of the Turkish War of National Liberation in 1922, the Ottoman Empire was in a constant state of war trying to prevent the “carving-up of the Turkish empire” by the Europeans, leaving the country economically exhausted and decimated due to the loss of much of its most productive lands and populations. During this time of turmoil and economic hardship, many in my grandparents’ and parents’ generation did not have the opportunity to obtain even an elementary education. Mustafa Kemal Ataturk led and won the War of Turkish National Liberation against the occupying European forces, a war that gave rise to the modern Turkish Republic. The new Republic gave priority to developing an education system available to all Turkish citizens. In a short period, schools were opened throughout the country, manned by teachers who were committed to Ataturk’s vision of an educated citizenry, idealistic about their country and optimistic about Turkey’s future. As a result, unlike my parents and grandparents, even in an underdeveloped, rural part of Turkey, I had access to excellent teachers and an excellent education that instilled in me pride in the history of the Turkish people and confidence that we could accomplish great things.

My mother was an illiterate woman who was the daughter of an Imam in a small village near Savur. Although she could not read or write, she was the most intelligent woman I have known. She was also very progressive and virtually worshipped Ataturk. It was at her insistence that all of her children got some degree of education. My father was the hardest working man I have ever known. He was, and still is, my role model. My oldest brother, Kenan, taught me how to read and write when I was 5 years old. Therefore, when I began school I was well ahead of my classmates. As importantly, Kenan was a role model for the pursuit of excellence and advancement through education and hard work. Kenan was the first of my family to attend college, specifically the Turkish Military Academy. Throughout his career, he was highly respected by his men and his colleagues for his fairness, hard work, and determination. He eventually rose to the rank of brigadier general in the Turkish Armed Forces.

Career decisions

I was the top student in my class throughout my primary education in Savur and my secondary education in Mardin. My favorite classes were math, Turkish, French and chemistry. In 10th grade an excellent chemistry teacher inspired me to become a chemist. However, academics were not my only love. Like every boy throughout most of the world, I grew up playing soccer. In high school, I played goal keeper for my high school (Mardin Lisesi), for Savur Spor (Savur), and for Mezopotamya Spor (Mardin). I was very good because I had fast reflexes and was fearless. More than once my teammates carried me off the field on their shoulders because I had made critical saves that helped win the game. During this period, I was asked by the Turkish Soccer Federation to participate in regional trials for the Turkish Under-18 team. Although playing for the Turkish National Team had long been a dream of mine, I chose not to participate in the trials because I thought that my height and weight were not sufficient for a national caliber player. Even though I quit playing soccer after the 10th grade, my love of the game remains, and I am an ardent supporter of Turkish and American national teams, the Galatasaray Professional Turkish soccer team, and the University of North Carolina–Chapel Hill Women’s Soccer Team.

Picture of the Mardin Lisesi high school soccer team.
Figure 3. Picture of the Mardin Lisesi high school soccer team in 1962. Aziz is the goal keeper (top row, 2nd from right).

When I graduated from high school I took the entrance exam for the B.Sc. Program in chemistry at Istanbul University and, at the suggestion of five of my friends from Mardin who were interested in becoming physicians, also took the Medical School Entrance Exam. I did well on both exams, but my friends prevailed on me to join them in medical sciences instead of continuing in Chemistry. I began medical school in November 1963.

Medical school

Coming to a cosmopolitan city like Istanbul had both advantages and disadvantages. I made friends with Turks of different ethnic backgrounds including Alevi, Armenian, Jewish, Greek, Kurdish, as well as the descendants of Turkish refugees from all of the Balkan countries. This enlightened my world view, especially with regard to the horrific effects of the Balkan Wars and World War I and the evil effects of religious and ethnic bigotry. Several of my professors, most of them Jewish, had fled Germany and nearby countries before or during World War II; despite the fact that many of them were leaders in their fields, they were rejected by many Western countries but were recruited to Turkish universities where they contributed to raising the education there to European standards. The Turkish nation owes a great debt of gratitude to these outstanding professors for their contributions to our science, education and even linguistics.

The Abdulgani and Meryem family in 1971
Figure 4. The Abdulgani and Meryem family in 1971. Aziz was abroad studying and is not in the photo. From back left to right: Kazım, Yıldız, Edibe, Meryem, Kenan, Abdul- gani, asemin, ahir (uncle), Nezihe. iddle: Hasan, eyran, Nurhan, Orhan, Zeynel, Sevim, Belma. Front: nieces and nephews.

The main disadvantage of attending the top medical school in Turkey was my fear of failure. Despite finishing at the top of my high school class in Mardin, I was now in class with fellow students who had graduated from some of the best public and private schools in Turkey. I was determined to show my classmates that a student from the “backward” southeast could succeed and even surpass students from more cosmopolitan areas. I decided that I could realize this goal by totally immersing myself in my studies to the exclusion of all else. I never went to a movie theater, concert, or play in Istanbul. My only diversion during that time was my involvement in the Turkish Nationalist Movement, which was opposing the Communist/Internationalist movement that was gaining strength in the country. I never participated in physical violence but strongly believed that the “comrades” who occupied the main administrative building of Istanbul University and hung the hammer-and-sickle red flag on top were wrong; I still believe that communism, as it is practiced, is evil.

Aziz with his medical school histology class
Figure 5. Aziz with his medical school histology class in 1964. Aziz is in the center next to his professor.

In my second year of medical school, I learned for the first time about the DNA double helix; I was fascinated and decided to become a biochemist when I graduated. My first thought was to begin research training as soon as possible, so in my final year of medical school I consulted the Chair of the Biochemistry Department, Mutahhar Yenson, about the possibility of joining the department upon graduation. He expressed the opinion that anyone obtaining a medical degree should practice medicine for at least two years before specializing in basic science research. So after graduation, at the top of my class, I returned to Savur to practice medicine in June of 1969.

Medical practice

For the first six months after I returned to Savur, I turned a room in my family’s house into a free clinic. Fortuitously, in the Fall of that year the Turkish Minister of Health passed through Savur, learned of my clinic, and suggested that I work for the Ministry of Health. Eventually, I was appointed Chief Medical Officer to a nearby village called Surgucu and was provided with a Jeep and a chauffeur. For the next year, I served people in Surgucu, in nearby villages and hamlets, and in a number of very remote villages. I was the first doctor that many of my patients had ever seen. I spent much of the salary I was paid by the Ministry of Health to buy drugs for my patients and toys for the small children whose families could not afford them. With simple medical procedures, I believe I saved the lives of many children.

One of the most challenging aspects of my medical practice was that some of my female patients spoke only Kurdish; during that period and in that part of Turkey, families did not send their daughters to school, so they did not learn Turkish. Local translators were usually men, and thus the women were often uncomfortable explaining intimate health problems to a man from their village. I tried to circumvent the problem by learning Kurdish, but I never became fluent. Nevertheless, I think the women appreciated the effort; they often kept the prescriptions I had written as a talisman after using the drugs I prescribed.

Looking back, I remember the 18 months that I practiced medicine as the happiest time in my life. However, I also found the practice of medicine intellectually frustrating; for example, I wanted to understand why streptomycin killed the tuberculosis bacterium, but penicillin did not. So throughout the time I practiced medicine, I also applied for fellowships to study biochemistry abroad.

PhD studies: cloning the photolyase gene

In 1971, I won a NATO fellowship to fund Ph.D. research in one of the member countries. I chose the United States, because it was the leader in scientific research in the world. I was admitted to the Department of Biochemistry Graduate Program at Johns Hopkins University and entered there in 1971. I was totally unprepared for the problems I would encounter there. Although, I had taken English classes during my final year of medical school, I could not communicate with my professors and fellow students. In addition, because of my previous academic success and patriotic upbringing, I was self-assured and confident to the point of arrogance, and people avoided me. It was like being in solitary confinement. As a result, I left Johns Hopkins in June of 1972 and returned to Savur to regroup. After practicing medicine again for about 6 months, and a brief detour to England, I returned to the United States more mature and reasonably proficient in English, and applied to Dr. Claud S. Rupert at the University of Texas at Dallas (UTD). I was accepted into the UTD Biology Program there in 1973 and joined Dr. Rupert’s lab in 1974.

Dr. Rupert is the scientist who discovered the enzyme photolyase; this discovery, in 1958, marks the beginning of the scientific field of DNA repair. In the bacterium E. coli, exposure to UV light kills the organism; however subsequent exposure to visible light reverses the killing effect. This is called photoreactivation, carried out by the enzyme photolyase. When I joined Dr. Rupert’s lab, the most outstanding question was “how does the enzyme absorb light.” To answer this question it was necessary to have the enzyme in large quantities and high purity, but no one had been able to purify it in sufficient amounts. About the time I joined the Rupert lab, molecular cloning was invented at Stanford University. I immediately saw the potential of this approach for solving the photolyase production problem. I would clone the E. coli photolyase gene, amplify the enzyme, and then purify it and characterize its chromophores and action mechanism.

Aziz with his Ph.D. mentor Claud Rupert
Figure 6. Aziz with his Ph.D. mentor Claud (Stan) Rupert in 2009.

The first step was to isolate a mutant defective in the photolyase gene so that I could use this mutant as a host for cloning. I devised a counter-intuitive experimental scheme to generate and select the mutant and performed the screen 1–2 times daily for 6 months before obtaining the first phr mutant. Along the way my self-confidence was challenged, not only by the difficulty of obtaining a mutant but also, during the period of repeated failures, by the comments of a labmate who told me that I did not have talent for lab research and should return to medical practice. The ultimate success of this experiment played a pivotal role in my evolution as a scientist because it required me to gather information from unrelated fields to create a method and because I persevered until the method worked. I believe that there are three characteristics essential for a successful scientist: creativity based upon knowledge, hard work, and perseverance in the face of failure. Although the paper describing this method has only been cited 6 times (including two self-citations), for me it is one of my most important papers because it gave me the confidence to carry on research and equally it helped convince Dr. Rupert that I was a good student so that he gave me the freedom to pursue my research goals.

Using the mutant I had isolated, I cloned the phr gene of E. coli in 1975, and began experiments to characterize the plasmid carrying the gene. However, in 1976 I was called back to Turkey to fulfill my military service obligation. I returned to Texas four months later with the rank of Second Lieutenant and resumed my work using the cloned gene to purify the enzyme. However, cloning a gene was such a major achievement at the time (I believe that phr was the first gene cloned east of the Rocky Mountains) that Dr. Rupert decided I had accomplished enough to earn a Ph.D. I started writing my doctoral dissertation in the spring of 1977 and, with the encouragement of Dr. Rupert, applied to three leading DNA repair labs. I did not receive an offer from any of them, probably because I had not published. I had been so engrossed in doing experiments that I had not taken the time to write up the 6–7 papers I had material for. Moreover, gene cloning was new and its utility was not appreciated yet by many in the field. Fortunately, I learned from a fellow graduate student that Dr. W. Dean Rupp of Yale University was planning on cloning the uvrA, uvrB, and uvrC genes responsible for nucleotide excision repair in E. coli. I applied to Dr. Rupp and, based upon Dr. Rupert’s strong personal recommendation, Dr. Rupp offered me a position in his lab. I defended in July of 1977 and left UTD in September to join Dr. Rupp’s lab, still not knowing how photolyase absorbs light.

Post-doctoral work: maxicells; dual incision I

When I joined the lab of Dr. Rupp, Yale University was one of the top three DNA research centers in the world and an exciting research environment. In addition to Dean Rupp, other pioneers in the field of repair and recombination there included Paul Howard-Flanders, Charles M. Radding and Franklin Hutchinson. I cloned the uvrA, uvrB, and uvrC genes in quick succession. While at Dallas, I had begun working on a method, which I called Maxicells, to identify the proteins encoded by cloned genes. At Yale, Dr. Rupp made suggestions to improve the method, which were crucial to its eventual success. It took almost a year to work out the details, but eventually the method worked. The paper describing Maxicells was published in 1979 and became an instant hit because it was applicable to identifying any plasmid-encoded protein. The method was widely used throughout the 1980s, and to this day it is my most cited research paper.

Having cloned the uvrA, uvrB, and uvrC genes, I used the Maxicell method with radioactive tracers to label, identify and purify the proteins encoded by these genes. Up to this point, the classical model for nucleotide excision repair was that a UV endonuclease incised the damaged strand 5′ to the damage and n exonuclease removed the damage in the 5′ to 3′ direction in the form of a 4–6 nucleotide fragment containing the damage. Much to my surprise, in the spring of 1982 I found that when I reconstituted the incision reaction in vitro using purified proteins, the UvrABC nuclease made concerted dual incisions, one 7 nucleotides 5′ to the dimer and the other 3–4 nucleotides 3′ to the dimer, releasing a 12–13 nucleotide long fragment carrying the dimer. I named the enzyme “ABC excinuclease” to emphasize the unique dual incision mechanism. This was a major discovery in the field of DNA repair; however because there were several other groups working hard on the same question, I could not tell anyone except a few lab colleagues about this result until we were ready to present it at a meeting and to publish it. Dr. Rupp presented the result for the first time at an international meeting on recombination and repair in France in the Spring of 1982. I still run into colleagues who say that this talk generated huge excitement at the meeting. Dr. Rupp’s talk was published in the meeting proceedings, and a full paper describing my work was published in 1983.

While I was in Dr. Rupp’s lab, other exciting events were also happening in my personal life. Back in Texas I had become a close friend of Gwen Boles, a graduate student in the same department at UTD. Gwen graduated three months before me and took a post-doctoral position in New York, working on the molecular basis of thalassemia. We continued to see each other on weekends when I moved to Yale, and we married in 1978. However, it was another 2 years before Gwen completed her work in New York, moved to Yale, and joined Dean Rupp’s lab to work on regulation of DNA repair genes in E. coli. Although living apart was not ideal, the additional time that Gwen spent in New York allowed her to eventually publish five papers from her post-doctoral work there.

Aziz and Gwen in 1994 in Chapel Hill
Figure 7. Picture of Aziz and Gwen in 1994 in Chapel Hill, NC.

In 1981, encouraged by my research successes, I began applying for faculty positions. I applied to about 50 universities and was turned down by all of them, some without even a reply to my application. Then I received a call from Mary Ellen Jones, the Chair of the Department of Biochemistry at the University of North Carolina at Chapel Hill. Dr. Jones was interested in recruiting molecular biologists to modernize the department. Gwen and I visited Chapel Hill, and we were both offered faculty positions in the spring of 1981. Because I was working on the reconstitution of ABC excinuclease and felt that I could not take a six-month break to set up a new lab, we accepted the positions on the condition that we could defer moving for a year. Dr. Jones agreed, and that enabled me to submit the paper describing the reconstitution and the dual incision mechanism in the fall of 1982, just before moving to Chapel Hill. This also allowed Gwen and me to write our first NIH grant proposal to work on photolyase. The proposal was funded, and as a result when we arrived in Chapel Hill most of our equipment was already in place, and we were able to start experiments three days after we arrived.

Aziz with Gwen and Rose
Figure 8. Aziz with Gwen and Rose in front of Ataturk Mausoleum, December 15, 2015.

Photolyase: “As complete as any research study can be”

When I started my own lab at UNC-CH, I decided to resume working on photolyase, specifically on identifying the chromophore and solving the action mechanism. In a relatively short period of time, we overexpressed and purified the enzyme and discovered that the enzyme has not one, but two cofactors, FADH– and MTHF that absorb light. In a series of experiments with collaborators from around the world, we found that MTHF acts as an antenna which absorbs light energy and transfers that energy to the FAD cofactor which carries out catalysis. Over the next 20 years, we and our collaborators defined the molecular mechanism in great detail and have traced all of the steps of the repair reaction in real time, from light absorbance to splitting of the dimer and return of the electron to the flavin cofactor. My work on photolyase has, with interruptions, spanned over 40 years and involved collaboration with numerous colleagues who were leaders in cofactor chemistry, flavin photochemistry, crystallography and ultrafast chemistry. It was therefore gratifying when a colleague recently wrote in a commentary on a paper we published in 2011 with our collaborator Dongping Zhong that “with this paper … the story of PL (photolyase), originating 62 years ago, has come to be as complete as any research study can be” [2].

Transcription-coupled repair; Yunus Emre destani

In 1985 and 1987, Philip Hanawalt and colleagues reported that transcription strongly stimulates nucleotide excision repair in human cells and in E. coli. They suggested that RNA polymerase stalled at a damage site increased the rate of damage recognition, which is the rate-limiting step in excision repair. We tested this model in vitro using purified E. coli proteins and found that RNA polymerase stalled at damage actually inhibited repair. From this we proposed that an additional factor recognized stalled RNA polymerase, displaced it from the damaged site, and simultaneously facilitated assembly of the excision nuclease at the damage. We identified and purified such a factor which we named TRCF (Transcription-Repair Coupling Factor). We went on to show that TRCF is the product of the mfd gene first described by Evelyn Witkin in 1956, and that purified TRCF, RNA polymerase, and ABC excision nuclease are sufficient to reconstitute transcription-coupled repair in vitro. I consider the paper describing this work to be our most aesthetically pleasing, both scientifically and stylistically. We made a hypothesis, obtained the necessary reagents to test it, and found the hypothesis to be correct. In the process we solved a mystery of 30 years standing (mutation frequency decline). The paper is well-written, states the problem concisely, and proceeds to describe the experimental results succinctly. The data is clear and unambiguous and the model has stood the test of time. To my Turkish colleagues who inquire about my research, this is my Yunus Emre Destani (Yunus Emre Opus), because Yunus Emre, a mystic poet who lived in the 14th century, is to the Turkish language what Chaucer is to the English language, and every Turk aspires to the perfection Yunus Emre achieved in his chosen field.

Excision repair in humans; Dual incision II (“Known only to God and me”); Molecule of the year

In 1987, I turned my attention to the mechanism of human nucleotide excision repair, which had remained poorly understood for over 20 years. We decided to pursue a biochemical approach to understand the reaction mechanism and focused initially on what we viewed as the most important question: do human cells utilize a UV endonuclease/exonuclease for excision or is there a dual incision mechanism similar to the one we had found in E. coli? For five years, we tried many assay systems, cell types, different cell extract preparations, and different types of substrates, to no avail. Finally on November 8, 1991 we captured the excised oligonucleotide: it was a 27-mer (“nominal 30-mer”) released by dual incisions. Yes, the mechanism was by dual incisions, but the dual incisions were different than E. coli. This discovery was one of the highlights of my research career. When I first saw the 27-mer, I told Gwen “there is an important biological fact about humans that is known only to God and me.” We followed up on this discovery, by isolating and purifying all of the proteins necessary for the dual incision reaction, and reconstituting the reaction in vitro from completely purified components. This work, combined with our elucidation of the mechanism of TCR, played an important role in the selection of DNA Repair as ‘Molecule of the Year’ by Science magazine in 1994. For this issue, Paul Modrich, Philip Hanawalt and I were asked to summarize the exciting discoveries in the field of DNA repair by our respective laboratories as well as those of a dozen other laboratories in the preceding year.

Brief map of the human genome; Piri Reis map

After the discovery of dual incisions in humans, we wanted to know the fate of the excised oligomer in human cells, but were unable to isolate the 30-mer from UV-irradiated human cells. After spending 20 years characterizing human excision repair in vitro, we finally captured the 30-mer produced in vivo. This has allowed us to map the sites of repair across the entire human genome at single nucleotide resolution. This repair map shows, in a geographic sense, repair mountains, valleys and canyons corresponding to regions of high, average, and low or no repair. This method will likely help us understand factors other than the primary repair proteins that affect repair efficiency and may have applications to improve chemotherapy. Personally, this is the most satisfying accomplishment in my lab in the last decade, and to Turkish colleagues I refer to it as “My Piri Reis Map.” Piri Reis was a Turkish admiral and cartographer who drew the world map in 1513 with a level of accuracy unrivaled by any other cartographer of his period. He is revered by Turks as a great scientist, arguably the last great scientist of the golden age of so-called “Islamic Science.” After submitting the paper describing this result, I went on a lecture tour of Peru and told Gwen that “if my plane hits the Andes and I die, I will die a happy man.”

DNA damage checkpoints

Cells respond to DNA damage by repairing it, by activating signal transduction pathways for arresting cell cycle progression, by changing the transcription profile, and by inducing apoptosis. These responses are important for cellular homeostasis and have been the subjects of detailed studies by many investigators. However, because of the very nature of the phenomena investigated, the biochemical analyses of these processes, with the exception of apoptosis, have been limited. With this general view, we decided to apply our experience in DNA repair to investigate the biochemistry of checkpoint activation. For the past 15 years, we have made significant contributions to the biochemistry of DNA damage checkpoints activated by UV damage. We developed several in vitro systems that captured specific steps in the signaling pathway. Perhaps our most physiologically relevant accomplishment has been the coupling of human nucleotide excision repair with the DNA damage checkpoint response in a completely defined system. I look at this work as the ultimate in reductionist biochemical research that aims to explain complex cellular phenomenon in a minimalist in vitro system.

Cryptochrome and the Circadian clock

Photolyase is not universally distributed in the biological world, and its presence in humans had been controversial for 35 years after its discovery in bacteria in 1958. In 1993 we conducted an exhaustive study on this subject and published a paper stating categorically that humans do not have photolyase. This result applied to both the classic photolyase that repairs cyclobutane-type pyrimidine dimers and another type of photolyase discovered by T. Todo that repairs pyrimidine (6–4) pyrimidone adducts. Then in 1995, Human Genome Sciences released the sequences for a number of partial human cDNAs, and among these was listed a photolyase homolog. We immediately obtained the cDNA for the entire gene, and shortly thereafter, discovered a second gene with high sequence similarity to photolyase. After cloning and expressing both genes, we found that neither of the recombinant proteins nor cells expressing the proteins had detectable photolyase activity of either type. We were still trying to decide what to do with these results, when “chance favored the prepared mind.”

In May of 1996, returning from a visit to Turkey, I read an article about the circadian clock and jetlag by Dr. William Schwartz in a flight magazine. I was most intrigued by the setting of the clock by blue light (wavelengths similar to those absorbed by the photolyase chromophores) and the fact that in some blind mice and people who lack conscious light perception, the circadian clock still responds to light because the “circadian visual system” is anatomically and physiologically distinct from the image-forming visual system. After reading this article I thought perhaps the human photolyase paralogs we had found might in fact be clock proteins that sense blue light. I discussed this with my lab and suggested that we call these proteins cryptochromes 1 and 2 (CRY1 and CRY2) in analogy with the plant blue light photoreceptors which also had sequence similarity to photolyase. The paper describing this work was published in Biochemistry in November of 1996, and it appears that it escaped the attention of the entire circadian clock community.

To test this claim I immediately set out to learn as much as I could about the circadian clock and neuroscience. By the end of 1997, we had shown that cryptochromes were highly expressed in the two anatomical locations critical to the clock, namely the ganglion cell layer of the retina, and the suprachiasmatic nucleus (SCN) in the brain, which is the neurological center of the clock in mouse and man. In particular, CRY1 mRNA exhibited high amplitude daily rhythmicity in the SCN. This was sufficient circumstantial evidence for us to publish a paper in PNAS claiming that the mammalian CRYs are circadian photoreceptors. This paper electrified the circadian clock community, but still we needed evidence of causality.

To prove our contention we had to show that mutations in the CRY genes altered the clock. We constructed a CRY2 mutant, and when it was tested in the laboratory of our collaborator Joseph Takahashi, it was apparent that even though the mutation affected sensitivity of the clock to light, even in complete darkness, it had an effect on the clock. We concluded, that CRY2 had both lightdependent and light-independent effects on the clock. In the meantime, our first paper on the potential role of CRY in the circadian clock led to the identification of a CRY homolog in Drosophila, and a Drosophila CRY mutant with greatly reduced photosensitivity was also isolated. Our work, also led to re-evaluation of Arabidopsis CRY mutants, and experiments performed by plant biologists showed that CRY also plays a role in the Arabidopsis circadian clock. Our CRY2 mutant mouse paper and the Drosophila and Arabidopsis papers were published within a week of one another. This, along with other important progress being made in the field, led to the circadian clock as runner-up for Science Magazine ‘Molecule of the Year’ in 1998.

Later in 1999, our group, in collaboration with T. Todo and J.S. Takahashi and another group of Dutch and Japanese colleagues, made mouse mutants defective in both CRYs and found that they no longer had a functioning circadian clock. There was rapid progress in the field, and by 2000 there was a reasonably detailed model for the clock in which CRY plays the role of the primary transcriptional repressor in the clock circuitry generated by a transcriptional and translational feedback loops. Current evidence indicated that CRY is primarily, if not exclusively, a repressor in mammals with no photoreceptor function, while in Drosophila it is the primary circulating photoreceptor. The discovery of cryptochrome as a circadian protein has given me a profound sense of gratitude and personal satisfaction for providing me the opportunity to contribute to an entirely different field of research from DNA repair and thus interacting with a new set of colleagues and a new way of thinking.

Full circle

For the past 15 years, we have been working on the mechanism by which CRY participates in the circadian clock in mammals and its photoreceptor function in Drosophila and have contributed to the current clock models for both organisms. Our work has also led us to discover that the circadian clock regulates excision repair in mice and that the carcinogenesis of UV light exhibits a circadian pattern. We are currently analyzing the circadian effect of repair in humans and the potential applications of this knowledge to chemotherapy regimens.

The Sancar lab
Figure 9. A recent photo of the Sancar lab (September 2015). From front right: Laura Lindsey-Boltz, Fazile Canturk, Aziz Sancar, Alana Oktay. From back right: Christopher Selby, Yanyan Yang, Yi-Ying Chiou, Jinchuan Hu, Michael Kemp, Muhammet Karaman, Wentao Li, Sheera Adar, Gulnihal Erkmen, Hiroaki Kawara.

Concluding remarks

I have had the good fortune of having parents who instilled a strong work ethic in me and a belief in the value of learning. I have been fortunate to have had excellent teachers throughout my education from primary school in Savur through high school in Mardin and medical school in Istanbul, and excellent mentors in graduate school and post-doctoral work in Texas and New Haven. I thank my family for their love. I am grateful to my wife, Gwen, for her love and support. In the words of one of my mentors, “Aziz, I don’t think you would have survived without Gwen.” I thank my goddaughter, Rose Peifer, who has added joy to my life and makes me feel young. Finally, I thank Gwen and Rose for keeping me on the straight and narrow.


1. Buckingham, J. (2004). Chasing the Molecule, Sutton Publishing, England

2. Stuchebrukhov, A. (2011). Watching DNA repair in real time. Proc Natl Acad Sci U S A 108, 19445–19446

From The Nobel Prizes 2015. Published on behalf of The Nobel Foundation by Science History Publications/USA, division Watson Publishing International LLC, Sagamore Beach, 2016

This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/ Nobel Lectures/The Nobel Prizes. The information is sometimes updated with an addendum submitted by the Laureate.

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Nobel Prizes and laureates

Eleven laureates were awarded a Nobel Prize in 2023, for achievements that have conferred the greatest benefit to humankind. Their work and discoveries range from effective mRNA vaccines and attosecond physics to fighting against the oppression of women.

See them all presented here.

Explore prizes and laureates

Look for popular awards and laureates in different fields, and discover the history of the Nobel Prize.