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Yeasts are eukaryotic unicellular microfungi that are widely distributed in the natural environment. Around 1000 yeast species are known, but this represents only a fraction of yeast biodiversity on Earth. The fermentative activities of yeasts have been exploited by humans for millennia in the production of beer, wine, and bread. The most widely exploited and studied yeast species is Saccharomyces cerevisiae, commonly referred to as ‘baker’s yeast’. This species reproduces asexually by budding and sexually by the conjugation of cells of opposite mating types. Other yeasts reproduce by fission (e.g., Schizosaccharomyces pombe) and by formation of pseudohyphae as in dimorphic yeasts, such as the opportunistic human pathogen Candida albicans. In addition to being widely exploited in the production of foods, beverages, and pharmaceuticals, yeasts play significant roles as model eukaryotic cells in furthering our knowledge in the biological and biomedical sciences. Several yeasts have had their genomes completely sequenced (e.g., S. cerevisiae in 1996; Sch. pombe in 2002), and research is under way to assign a physiological function to sequenced yeast genes. The study of yeasts not only provides insights into how a simple eukaryote works but also leads to understanding of several human diseases and heritable disorders.