STKs (serine/threonine-protein kinases) catalyse the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. TSSK (testis-specific serine kinase) proteins are almost exclusively expressed postmeiotically in the testis and play important roles in spermatogenesis and/or spermiogenesis. There are five mammalian TSSK proteins which show differences in their localization and timing of expression []. TSSK3 has been reported to be expressed in the interstitial Leydig cells of adult testis. Its mRNA levels is low at birth, increases at puberty, and remains high throughout adulthood [, ].
