The male germline arises from primordial germ cells (PGCs) in the embryo which migrate into the developing gonad and subsequently undergo mitotic arrest. A remarkable feature of the resulting prospermatogonia is the acquisition of an adult tissue stem cell phenotype (formation of spermatogonial stem cells - SSCs) and loss of the pluripotent phenotype characteristic of PGCs, which happens late in gestation around the time of mitotic arrest. In the postnatal mammalian testis, some prospermatogonia will become bona fide SSCs, while others contribute to the first wave of spermatogenesis, and still others undergo apoptosis and are lost. In collaboration with Dr. John McCarrey, we are studing the molecular mechanisms responsible for specification of the SSC pool during embryonic and postnatal testis development.