Acts as a negative regulator of entry into mitosis (G2 to M transition) by phosphorylation of the CDK1 kinase. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Observed in the nucleus during prophase and metaphase. In the 12-13-cell embryo, expressed in the E blastomere. In the 16-cell embryo, expressed in the eight AB cells. Expression is first seen in the 12-13-cell embryo and is last observed in the 16-cell embryo before division. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily.