Aziz Sancar

He worked out how nucleotide excision repair cuts damaged DNA, showing in 1983 that purified UvrA, UvrB and UvrC make dual incisions around a UV lesion, and later reconstituted the human reaction; he shared the 2015 Nobel Prize in Chemistry with Tomas Lindahl and Paul Modrich for the mechanisms of DNA repair.

As a doctoral student he cloned the gene for photolyase, the enzyme that repairs UV damage with visible light, and later showed that it carries two light-absorbing cofactors and works through a flavin electron-transfer cycle.

His lab found two human genes related to photolyase, which proved to be cryptochromes, and with collaborators showed they are essential parts of the mammalian circadian clock.

To clone the photolyase gene he first needed a mutant lacking it; the screen failed on the first attempts and he reports it took about 11 months to isolate one.

This is consistent with a researcher who kept repeating a difficult experiment until it worked, though the account is largely his own. Persistence

After his group reported in 1993 that humans lack photolyase, photolyase-like human genes turned up in 1995; they had no repair activity, and he proposed they were clock proteins and tested that idea.

This is consistent with a scientist who reinterpreted his own earlier conclusion when new genes appeared, as told in his own account. Belief Updating

In 1996 he proposed that the two photolyase-like human proteins be named cryptochromes and tested whether they act in the circadian clock, moving his lab from DNA repair into clock biology.

This is consistent with a researcher who extended his work into a neighbouring field and learned it from scratch. Cross-Domain Range