Illustration of the US geneticist Leland Hartwell (born 1939). Hartwell is best known for his research into the regulation of the cell cycle. Working with bakers yeast (Saccharomyces cerevisiae) Hartwell identified 100 genes that regulated the cell cycle, the sequence of events that lead to a cell dividing into two daughter cells. He also discovered that there were checkpoints built into the cycle where DNA (deoxyribonucleic acid) damage was checked for and either repaired or the cell cycle halted. Defects in these checkpoints can lead to cancer. Hartwell received a share of 2001 Nobel Prize in Physiology or Medicine, along with Paul Nurse and Tim Hunt, for this work.