Prohibitin is an evolutionarily conserved gene with homologues found in organisms ranging from yeast to man. In man the gene is located on chromosome 17 at q21. The deduced amino acid sequences of the protein products from mouse and rat are identical; and these differ from the human protein sequence by a single conserved amino acid. Prohibitin has antiproliferative activity and available data suggest a role in such diverse processes as normal cell cycle regulation, replicative senescence, cellular immortalization, and the development of sporadic breast tumors. Although its functional activity is presently unknown, the 30,000-Da protein has been located in the inner membrane of mitochondria, where it is postsynthetically modified, as well as on the plasma membrane of B cells, where it is associated with the IgM receptor. Prohibitin's evolutionary conservation and ubiquitous expression indicate that it is a fundamentally important gene; and current data suggest a functional role in such dissimilar processes as development, senescence, and tumor suppression.