Spontaneous mutations occur in the DNA as a result of endogenous cellular processes. The antimutagenic processes within a cell consist primarily of mechanisms of DNA repair, which are critical for maintenance of genomic stability, while mutagenic processes include mistakes by the replicative machinery and spontaneous alterations in the base chemistry of DNA. In Saccharomyces cerevisiae spontaneous mutagenesis assays are typically employed when studying the DNA damage repair pathways, since loss of one of these mechanisms results in a detectable increase in the spontaneous mutation rate, which is determined by first growing cells to log phase, then subculturing them to a very low concentration and incubating for several days. This allows for many cell divisions and thus many opportunities for mutations to occur in the genome. The selection of mutants is typically based on a specific genetic marker such as an auxotrophic marker, and the total number is compared to the total number of viable cells in order to determine the mutation rate for an exponentially growing culture.