American biologist who, with Sydney Brenner (Brenner, Sydney) and John E. Sulston (Sulston, John E.), won the Nobel Prize for Physiology or Medicine in 2002 for their discoveries about how genes regulate tissue and organ development via a key mechanism called programmed cell death, or apoptosis.

Horvitz received a B.A. (1972) and a Ph.D. (1974) from Harvard University. In 1978, after a stint with Brenner at the Medical Research Council in England, he moved to the Massachusetts Institute of Technology, where he became a full professor in 1986.

In the 1970s Horvitz began his prizewinning work on programmed cell death, a process that is essential for normal development in all animals. During fetal development of humans, huge numbers of cells must be eliminated as body structures form. For example, programmed cell death sculpts the fingers and toes by removing tissue that was originally present between the digits. Likewise, it removes surplus nerve cells produced during early development of the brain. In a typical adult human, about one trillion new cells develop each day; a similar number must be eliminated to maintain health and to keep the body from becoming overgrown with surplus cells.

Horvitz's research focused on determining if a specific genetic program controls cell death. His studies centred on the nematode Caenorhabditis elegans, a near-microscopic soil worm that had been identified by Brenner as an ideal organism on which to study programmed cell death. In 1986 Horvitz reported the first two “death genes,” ced-3 and ced-4, which participate in the cell-killing process. Later he showed that another gene, ced-9, protects against cell death by interacting with ced-3 and ced-4. Horvitz also established that humans have a counterpart ced-3 gene. Scientists later found that most of the genes involved in controlling programmed cell death in C. elegans have counterparts in humans. Such knowledge about programmed cell death contributed to important advances not only in developmental biology but also in medicine, especially concerning cancer treatments.