Between 2% and 12% of couples worldwide are affected by reduced fertility. Men who have defects in sperm production (spermatogenic defect) account for about half of these cases. In Drosophila and mouse, targeted disruptions of numerous sterility- associated genes have been created. Physiological studies in the Drosophila and in mouse also indicate that spermatogenesis is subjected to complex regulation, and male infertility may result from aberrant regulatory events. In the human being, deletions of the Y chromosome account for only 10% of cases with spermatogenic defect, and etiologies of remaining 90% of cases are still unknown. It is evident that multiple genes are involved in male infertility. For cases with severe spermatogenic defect , testicular histology shows either decreased number of germ cells in all developmental stages (hypospermatogenesis) or complete absence of germ cells (Sertoli cell only syndrome or SCOS). It appears that there is an intrinsic defect which causes depletion of germ-line stem cells (spermatogonia) for cases with hypospermatogenesis or SCOS. Of 25 genes exclusively expressed in mouse spermatogonia, 3 are Y-linked, 10 are X-linked, and only 12 are distributed on autosomes.