What is "synthetic lethality"

n. (context genetics English) A type of genetic interaction in which two (or more) mutations which are not lethal when occurring singly in a cell are lethal when they occur together in the same cell.

Synthetic lethality arises when a combination of mutations in two or more genes leads to cell death, whereas a mutation in only one of these genes does not, and by itself is said to be viable. In a synthetic lethal genetic screen, it is necessary to begin with a mutation that does not kill the cell, although may confer a phenotype (for example, slow growth), and then systematically test other mutations at additional loci to determine which confer lethality. Synthetic lethality indicates functional relationships between genes.