TSSK3 (testis specific serine kinase 3) is a serine/threonine protein kinase essential for male fertility and spermatogenesis 1. The protein localizes to the sperm tail and is expressed in elongating sperm during spermiogenesis 1. TSSK3 exhibits optimal enzymatic activity at 30°C, consistent with its testis-specific function, and can autophosphorylate as well as phosphorylate substrates including histones and casein 2. The kinase is regulated through phosphorylation of threonine 168 in its T-loop by phosphoinositide-dependent kinase-1 (PDK1), which increases its catalytic activity 2. TSSK3 knockout mice are completely sterile with dramatically reduced sperm counts, abnormal sperm morphology, and disorganized seminiferous epithelium 13. Phosphoproteomic analysis reveals that TSSK3 is crucial for phosphorylation of multiple proteins implicated in male infertility, including GAPDHS, ACTL7A, ACTL9, and REEP6 1. The essential role in spermiogenesis and exclusive testicular expression make TSSK3 a potential target for nonhormonal male contraception 3. Expression is post-meiotic and confined almost exclusively to testis across mammalian species 45.