Despite being only one-tenth the size of a white blood cell, many of the cellular functions of higher species are present in unicellular yeast. Two species of yeast have been pressed into service as model organisms: S. cerevisiae and its distant cousin S. pombe. Each one has nearly 200 genes homologous to human genes involved in disease, with 23 for cancer alone. Used since Ancient Egypt, yeast has economic importance in beer and bread making.

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