American scientist who, with Sir Paul M. Nurse (Nurse, Sir Paul M.) and R. Timothy Hunt (Hunt, R. Timothy), shared the Nobel Prize for Physiology or Medicine in 2001 for discovering key regulators of the cell cycle.

Hartwell studied at the California Institute of Technology (B.S., 1961) and the Massachusetts Institute of Technology (Ph.D., 1964). He served on the faculty of the University of California at Irvine from 1965 to 1968, when he moved to the University of Washington. In 1996 he joined the Fred Hutchinson Cancer Research Center in Seattle, Washington, becoming president and director in 1997.

In the late 1960s Hartwell began using baker's yeast to study how cells control their growth and division. He identified more than 100 genes (gene), termed cell-division-cycle (CDC) genes, involved in cell-cycle control. One such gene, named cdc28, was demonstrated to control the first phase and so became known as “start.” Hartwell also found that the cycle includes optional pauses, called checkpoints, that allow time for repair of damaged DNA. His work helped expand scientific understanding of cancer and other diseases that occur when the machinery of the cell cycle goes awry.