May be involved in a signaling pathway during male germ cell development or mature sperm function. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by phosphorylation on Thr-168, potentially by autophosphorylation. Developmentally expressed in testicular germ cells. In adult testis, expression was detected in round and condensing spermatids, but not in meiotic pachytene spermatocytes. Not expressed in brain, ovary, kidney, liver or early embryonic cells. Expression begins 20-24 days after birth and is maximal in the adult. The pattern of expression suggests that STK22D is expressed postmeiotically. Autophosphorylated. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. PubMed:10781952 has termed the gene 'STK22D' as it was then thought that there were two different closely related genes.