In 2015, Aziz Sancar, a Turkish professor at the University of North Carolina, won the Nobel Chemistry Prize along with Tomas Lindahl and Paul Modrich, for their work on how cells repair damaged DNA. Other Turkish scientists include physician Hulusi Behçet who discovered Behçet's disease and mathematician Cahit Arf who defined the Arf invariant. Turkey was ranked 51st in the Global Innovation Index in 2020, it has increased its ranking considerably since 2011, where it was ranked 65th.