Although epithelial stem cells are implicated in the etiology of both benign prostatic hyperplasia and cancer (1 ,2 ), there has, until recently, been little information regarding their characteristics. Stem cells are well characterized in several other mammalian epithelial tissues, such as the lining of the gut (3 ) and the epidermis of the skin (4 –6 ). In both of these tissues, there is a small stem cell population that divides rarely to produce progeny that are either stem cells or transit amplifying cells. The latter are capable of rapid division but have a limited proliferative capacity and will ultimately produce fully differentiated, nonproliferative daughters.