TSSK4 (testis specific serine kinase 4) is a serine/threonine kinase that plays critical roles in male reproduction through regulation of spermatogenesis and sperm function. The protein maintains its kinase activity through autophosphorylation at Thr-197 in the T-loop region 1. TSSK4 is expressed from haploid round spermatids to mature spermatozoa and localizes to individualization complexes during spermiogenesis 12. The kinase functions by phosphorylating key substrates including CREM at Ser-117, which regulates testis gene expression 3, and proteins involved in microtubule organization and flagella structure that coordinate sperm motility 2. TSSK4 also induces apoptosis in a kinase-dependent manner, contributing to normal spermatogenesis by eliminating defective germ cells 4. Genetic variants in TSSK4 are associated with male infertility, with specific polymorphisms linked to impaired spermatogenesis in Chinese populations 5. Beyond reproductive function, TSSK4 expression is regulated by ATF6 and plays unexpected roles in pulmonary fibrosis by phosphorylating HSP90β and restricting AKT survival signaling 67. The protein's diverse functions highlight its importance in both reproductive biology and cellular homeostasis.