Gene is inviable only in certain genetic constitutions. Thus, two single mutations have no or insignificant phenotypic consequence separately but the double mutant may be lethal. In yeast, only ∼1000 of the genes are essential and 5000 are viable even when deleted. The synthetic lethals cells include more than one defective gene. Partners in synthetic lethal systems can be essential as well as non-essential genes. In gene networks, at least one of the genes must be essential although most of the non-essential genes can compensate for each other. Synthetic lethals may be involved in inbreeding depression and may be the cause of some hybrid inviabilities and sterility. A synthetic lethal test may be used in anticancer drug design by combining two different mutations that in combination, seriously impair cells to gain information on how to kill cancer cells.  inbreeding,  synthetic genetic array,  synthetic genetic networks,   Saccharomyces cerevisiae ; Jacobson MD et al 2001 Genetics 159:17; Davierwala AP 2005 Nature Genet 37:1147.