TSKS, testis-specific serine kinase substrate, is expressed in the testis and is downregulated in cancerous testicular tissue, in comparison with adjacent normal tissue. TSKS expression is very low to undetectable in seminoma, teratocarcinoma, embryonal, and Leydig cell tumors, while high in testicular tissue adjacent to tumors which contain pre-malignant carcinoma in situ. Recently it has been shown in human testis to be localized to the equatorial segment of ejaculated human sperm. The finding of a TSKS family member in mature sperm suggests that this family of kinases might play a role in sperm function. TSKS is localized during spermiogenesis to the centrioles of post-meiotic spermatids, where it reaches its greatest concentration during the period of flagellogenesis.