Plays a role in the organization of the formation of the main body axis of developing embryo. Acts as an inhibitor of differentiation of primary neurons. Inhibits the ability of ectopically expressed NEUROD1 and other bHLH factors to promote early retinal cell differentiation. May participate in the Wnt signaling pathway. May regulate the circadian clock via phosphorylation of the major clock components. ATP + L-seryl-[tau protein] = ADP + H(+) + O-phospho-L-seryl-[tau protein] ATP + L-threonyl-[tau protein] = ADP + H(+) + O-phospho-L-threonyl-[tau protein] Expressed in the unfertilized egg and throughout the early embryonic stages. Expressed in the neural plate at stage 13. Expressed throughout the eye primordia and cranial ganglia at stage 25. Highly expressed in the differentiated retinal ganglion cell layer at stage 40. Phosphorylated. Activated by phosphorylation at Tyr-216. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. GSK-3 subfamily.