To identify new genes in an organism, a genetic approach can be used to screen for mutations that display a particular phenotype. Genotoxic agents, such as ultraviolet (UV) light, ionizing radiation, or chemicals can be used to randomly induce DNA lesions in the genome. Most efficient mutagenesis occurs when a mutagen confers a high frequency of mutations with low lethality, in the range of 10 to 50% survival. These mutations can be in the form of frameshifts, deletions, or rearrangements. To initiate a mutagenesis, a fresh subculture of cells grown into log phase is collected, washed, and resuspended in potassium phosphate buffer. The mutagen is added to the culture for a predetermined time, deactivated, and washed from the cells. The cells are allowed to recover from the treatment by incubating in liquid or on solid medium. Mutants can be isolated by screening individual colonies or by using direct selection of cells from the mutagenized cell population.