Acts as a negative regulator of entry into mitosis (G2 to M transition) (PubMed:8573790, PubMed:15581871). This kinase specifically phosphorylates and inactivates cyclin B1-complexed CDC2 (PubMed:8573790, PubMed:15581871). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Binds 2 magnesium ions per subunit. Negatively regulated by phosphorylation in the M-phase. Expressed in embryos; expression remains high in the proliferating cells of the central nervous system well after cells in the rest of the embryo have ceased dividing. Expressed both maternally and zygotically during postblastoderm divisions of embryogenesis. Phosphorylated during M and G1 phases. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily.