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''Saccharomyces'' is considered a yeast, although this term is historical and ill-defined. ''Saccharomyces'' is a genus of fungus including many species. The distinct species of ''Saccharomyces'' are revised frequently as more research is done. All species are unicellular and capable of fermentation. ''Saccharomyces cerevisiae'' is the most well known species of yeast. It is used in the fermentation of beer, wine, and sake, and as a leavening agent in bread. It is commonly referred to as "ale yeast", "wine yeast", or "bread yeast". ''S. pastorianus'', known as lager yeast, is a hybrid closely related to ''S. cerevisiae'' but is not a true species. ''S. cerevisiae'' is commonly studied as a model organism and was the first eukaryote to have its genome entirely sequenced.