In our previous study, a novel human testis-specific gene, SPATA12, was identified using the digital differential display program. In the current study, both SPATA12 mRNA and protein levels in the developmental stage of the testis were detected by SYBR real-time reverse transcriptase polymerase chain reaction and Western blot. A high level of SPATA12 gene expression was observed in normal adult testis but was completely absent in fetal testis. Both in situ hybridization and immunohistochemical analysis showed that SPATA12 was expressed in seminiferous tubules of adult testis-more precisely in spermatocytes, spermatids, and spermatozoa-but there was no expression in Sertoli and Leydig cells. These results showed that SPATA12 is a stage-specific and germ cell-specific gene, which suggests that it must be involved in the development of testicular maturation. It was also found that the expression level of SPATA12 mRNA in the testes of infertile men was associated with the amount and density of germ cells. With a decrease in the number of germ cells, the expression of SPATA12 mRNA was lower. In addition, the signal in the testes of patients with cryptorchidism or Sertoli cell only syndrome was not detected. Flow cytometry analysis of SPATA12 in human HeLa cells and mouse GC-1 spg germ cells indicated that the expression of the SPATA12 gene may delay the progression of G(1) to S in the cell cycle. SPATA12 was also shown to be lost in testicular germ cell tumors both at the level of transcription and translation. We hypothesized that the putative function of SPATA12 is to maintain the cell in a differentiated state and/or to suppress cell proliferation.