Hartwell, Leland Harrison (1939-), an American geneticist, advanced understanding of the fundamental principles that govern cell division through his studies of yeast-cell replication. For his work, he shared the 2001 Nobel Prize in physiology or medicine with British biologists Timothy Hunt and Paul Nurse .

Hartwell attended high school in Los Angeles and then enrolled in Glendale Junior College, where his interest in science was encouraged by his teachers. He earned his undergraduate degree from the California Institute of Technology in 1961 and his doctorate in microbiology from the Massachusetts Institute of Technology in 1964.

In 1965, Hartwell became an assistant professor at the University of California at Irvine. He decided to study yeast cells to try to find genes that control cell development. He wanted to see if a cell's lack of control over certain points in its division might contribute to cancer. Using baker's yeast as a model organism, Hartwell discovered more than 50 genes that controlled the cycle of cell division and multiplication in yeast. One, the start gene, controlled the first step in the cell cycle.

In 1968, Hartwell became an associate professor at the University of Washington. He and his associates were able to find human counterparts to the genes they had studied in yeast. They also found that some genes, which they called checkpoints, made sure each step of cell division was complete before the next one started. His research prompted investigation into novel therapies for use against cancer and other diseases.

Hartwell was appointed professor of genetics in 1973. In 1997, he became president and director of the Fred Hutchinson Cancer Research Center in Seattle. He was elected a member of the National Academy of Sciences in 1987 and has received numerous other honors, including the 2000 Massry Prize. In 2001, he shared the Nobel Prize in physiology or medicine.