TSKS (testis-specific serine kinase substrate) is a 592-amino acid protein expressed exclusively in testicular tissue that plays a critical role in male germ cell development. 1 Mechanistically, TSKS functions as a substrate for testis-specific serine/threonine kinases TSSK1 and TSSK2, localizing specifically to centrioles of spermatids during flagellogenesis and persisting in mature spermatozoa. 2 The TSSK2/TSKS kinase-substrate complex is essential for spermiogenesis, as targeted deletion of Tssk1/2 causes male infertility with morphological defects including failure of spermatid elongation and increased apoptosis. 3 Phosphorylation of TSKS at serine residues (Ser281/285) by TSSK2 has been demonstrated both in vitro and in vivo. 34 Recently, TSKS was identified as an interacting partner of STK33, another kinase crucial for spermiogenesis, suggesting broader roles in germ cell regulation. 5 Clinically, TSKS expression is downregulated in testicular tumors compared to normal tissue, 1 making it a potential biomarker for testicular cancer. The tissue-specific expression and post-meiotic localization pattern position TSKS as a candidate target for reversible male contraception development. 4