TSSK6 (testis-specific serine kinase 6) is a testis-specific protein kinase that is highly conserved in mammals and essential for spermatogenesis. In spermatozoa, TSSK6 is expressed after meiosis and is localized to the nucleus of spermatocytes and to the posterior part of the head of spermatocytes. TSSK6-deficient spermatozoa exhibit a defective histone-to-fischerin transition (required for chromatin remodeling after meiosis), as well as a reduction in actin polymerization in spermatocytes. The result is striking morphological defects in the spermatozoa, including nuclear malformations, a hairpin shape with the head facing backward, or complete separation of the head. In addition, Tssk6 is frequently aberrantly expressed in colorectal cancer and exhibits oncogenic activity.