The tyrosine-kinase receptor c-kitand its ligand, stem cell factor (SCF), are essential for the maintenance of primordial germ cells (PGCs) in both sexes. However, c-kitand a postmeiotic- specific alternative c-kitgene product play important roles also during post-natal stages of spermatogenesis. In the adult testis, the c-kitreceptor is re-expressed in differentiating spermatogonia, but not in spermatogonial stem cells, whereas SCF is expressed by Sertoli cells under FSH stimulation. SCF stimulates DNA synthesis in type A spermatogonia cultured in vitro, and injection of anti-c-kitantibodies blocks their proliferation in vivo. A point mutation in the c-kitgene, which impairs SCF-mediated activation of phosphatydilinositol 3-kinase, does not cause any significant reduction in PGCs number during embryonic development, nor in spermatogonial stem cell populations. However males are completely sterile due to a block in the initial stages of spermatogenesis, associated to abolishment of DNA-synthesis in differentiating A1-A4spermatogonia. With the onset of meiosis c-kitexpression ceases, but a truncated c-kitproduct, tr-kit, is specifically expressed in post-meiotic stages of spermatogenesis, and is accumulated in mature spermatozoa. Microinjection of tr-kit into mouse eggs causes their parthenogenetic activation, suggesting that it might play a role in the final function of the gametes, fertilization.