Although genetic analyses and transformation can be performed with a number of taxonomically distinct varieties of yeast, extensive studies have been limited primarily to the many freely interbreeding species of the budding yeast Saccharomyces and to the fission yeast Schizosaccharomyces pombe. Although "Saccharomyces cerevisiae" is commonly used to designate many of the laboratory stocks of Saccharomyces used throughout the world, it should be pointed out that most of these strains originated from the interbred stocks of Winge, Lindegren, and others who employed fermentation markers not only from S. cerevisiae but also from S. bayanus, S. carlsbergensis, S. chevalieri, S. chodati, S. diastaticus, etc. Nevertheless, it is still recommended that the interbreeding laboratory stocks of Saccharomyces be denoted as S. cerevisiae, in order to conveniently distinguish them from the more distantly related species of Saccharomyces.

Care should be taken in choosing strains for genetic and biochemical studies. Unfortunately there are no truly wild-type Saccharomyces strains that are commonly employed in genetic studies. Also, most domesticated strains of brewers’ yeast and probably many strains of bakers’ yeast and true wild-type strains of S. cerevisiae are not genetically compatible with laboratory stocks. It is often not appreciated that many "normal" laboratory strains contain mutant characters. This condition arose because these laboratory strains were derived from pedigrees involving mutagenized strains, or strains that carry genetic markers. Many current genetic studies are carried out with one or another of the following strains or their derivatives, and these strains have different properties that can greatly influence experimental outcomes: S288C; W303; D273–10B; X2180; A364A; S1278B; AB972; SK1; and FL100. The haploid strain S288C (MATa SUC2 mal mel gal2 CUP1 flo1 flo8-1 hap1) is often used as a normal standard because the sequence of its genome has been determined (Goffeau et al., 1996), because many isogenic mutant derivatives are available, and because it gives rise to well-dispersed cells. However, S288C contains a defective HAP1 gene, making it incompatible with studies of mitochondrial and related systems. Also, in contrast to S1278B, S288C does not form pseudohyae. While true wild-type and domesticated bakers’ yeast give rise to less than 2% r ^- colonies (see below), many laboratory strains produce high frequencies of r ^- mutants. Another strain, D273–10B, has been extensively used as a typical normal yeast, especially for mitochondrial studies. One should examine the specific characters of interest before initiating a study with any strain. Also, there can be a high degree of inviability of the meiotic progeny from crosses among these "normal" strains.

Many strains containing characterized auxotrophic, temperature-sensitive, and other markers can be obtained from the Yeast Genetics Stock Culture Center of the American Type Culture Collection (http://www.atcc.org/SearchCatalogs/YeastGeneticStock.cfm), including an almost complete set of deletion strains (http://www-deletion.stanford.edu/cgi-bin/deletion/search3.pl.atcc). Currently this set consists of 20,382 strains representing deletants of nearly all nonessential ORFs in different genetic backgrounds. Deletion strains are also availabe from EUROSCARF (http://www.uni-frankfurt.de/fb15/mikro/euroscarf/col_index.html) and Research Genetics (http://www.resgen.com/products/YEASTD.php3). Other sources of yeast strains include the National Collection of Yeast Cultures (http://www.ncyc.co.uk/Sacchgen.html) and the Centraalbureau voor Schimmelcultures (http://www2.cbs.knaw.nl/yeast/webc.asp). Before using strains obtained from these sources or from any investigator, it is advisable to test the strains and verify their genotypes.