Recently we have identified a novel gene mina53 (mina), which is a direct transcriptional target of oncoprotein Myc. Mina53 protein was shown to be highly expressed in tumour cells and to play a role in cell proliferation. Here we report the expression of Mina53 in mouse testis, which contains proliferating cells and expresses many cancer-related genes. Immunohistochemical studies by using newly produced monoclonal antibody to Mina53 showed that Mina53 was expressed in the nuclei of spermatogonia. Mina53 was also expressed in meiotic prophase cells such as preleptotene, leptotene and zygotene, and weakly in early pachytene spermatocytes, but was absent in late pachytene spermatocytes, spermatids and mature sperm. The expression pattern of Mina53 was quite similar to that of proliferation cell nuclear antigen (PCNA). Using experimental cryptorchid testis, it was found that Mina53 was highly expressed in undifferentiated spermatogonia, which were PCNA-positive. These results suggest that Mina53 is prominently expressed in proliferating, undifferentiated spermatogonia, and plays a role in cell proliferation from the spermatogonial stage to the meiotic prophase in spermatogenesis, but not in meiotic divisions per se.