Acts as a negative regulator of the normal cell cycle progression. May function in regulating proliferation by the phosphorylation and subsequent plasma membrane targeting of galactosyltransferase (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Present throughout the early embryo. In late embryos levels are highest in the CNS. Expressed both maternally and zygotically. Highest levels in early embryogenesis (0-6 hours), low levels during later embryogenesis, moderate levels in pupae and adults. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily. Intron retention.