Loss-of-function mutation of the Kit gene causes a severe defect in spermatogenesis that results in infertility due to the inability of its cognate ligand, KIT ligand (KITL), to stimulate spermatogonial proliferation and differentiation. Although self-renewal of mouse spermatogonial stem cells (SSCs) depends on glial cell line-derived neurotrophic factor (GDNF), there is no unequivocal evidence that SSCs with a KIT deficiency can self-renew in vivo or in vitro. In the testis of W(v)/W(v) mice, in which the KIT tyrosine kinase activity is impaired, spermatogonia with SSC phenotype were identified. When W(v)/W(v) spermatogonia were cultured in an SSC culture system supplemented with GDNF in a 10% O(2) atmosphere, they formed clumps and proliferated continuously. An atmosphere of 10% O(2) was better than 21% O(2) to support SSC self-renewal. When W(v)/W(v) clump-forming germ cells were transplanted into testes of infertile wild-type busulfan-treated mice, they colonized the seminiferous tubules but did not differentiate. However, when transplanted into the testes of infertile W/W(v) pups, they restored spermatogenesis and produced spermatozoa, and progeny were generated using microinsemination. These results clearly show that SSCs exist in W(v)/W(v) testes and that they proliferate in vitro similar to wild-type SSCs, indicating that a functional KIT protein is not required for SSC self-renewal. Furthermore, the results indicate that a defect of KIT/KITL signaling of W(v)/W(v) SSCs does not prevent spermatogonial differentiation and spermatogenesis in some recipient strains.