Serine/threonine-protein kinase which is involved in spermatogenesis (PubMed:12019230, PubMed:8417991, PubMed:29879108). In spermatocytes, regulates meiosis and the localization and assembly of major sperm protein (MSP) into fibrous bodies (PubMed:12019230, PubMed:8417991). In addition, may suppress the initiation of spermiogenesis downstream of spe-8, spe-12, spe-27 and spe-29 (PubMed:12019230). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family.