A cDNA clone denoted SPK (spermatogenesis-related protein kinase) was isolated from human fetal brain cDNA library. The deduced protein was 99% identical to human PDZ-binding kinase (PBK), which is a mitotic kinase. Its isoform, which is about 300 bp longer at 3' end, was also isolated in this study. SPK gene was assigned to human chromosome 8p21.2 by radiation hybrid. Northern blot analysis indicated that SPK mRNA was predominantly expressed in testis. SPK mRNA was localized to the outer cell layer of seminiferous tubules by in situ hybridization, suggesting involvement of it in the process of spermatogenesis.