A mouse homologue of Drosophila germ cell-less, mouse germ cell-less-1 (mgcl-1), is highly expressed in the testis. Previous report revealed that the fertility of the mgcl-1(-/-) male mice is reduced significantly as a result of various morphological abnormalities in the sperm (Kimura et al., 2003). To elucidate the function of mgcl-1 in spermatogenesis, the expression of mGCL-1 in the wild-type testis was examined. Immunohistochemical studies demonstrated that mGCL-1 first appeared in the nuclei of the pachytene spermatocytes at stage VI of the seminiferous epithelium, and existed in those of spermatids until step 8 during spermatogenesis. mGCL-1 was not detectable after step 9 spermatids. The testicular cells and epididymal sperm were further analyzed morphologically using mgcl-1(-/-) mice. In the testis, deformed nuclei first occurred in the pachytene spermatocytes at stage VI, which is consistent with the time of the first appearance of the mGCL-1 protein in the wild-type testis. Abnormal nuclei and acrosomes were found in spermatids after step 5, and nuclei of the spermatids and epididymal sperm were frequently invaginated. In addition, variously deformed sperm such as bent-neck, multi-headed or multi-nucleated sperm were observed in the mgcl-1(-/-) cauda epididymidis. However, several key structures such as the acroplaxome marginal ring (Kierszenbaum et al., 2003), postacrosomal sheath, and posterior ring apparently formed. In addition, MN7 and MN13, essential substances for fertilization that are located in sperm heads, were detectable in the mgcl-1 null sperm. These observations provide important insights into the mechanisms regulating the nuclear architecture and causes of human infertility.