Acts as a zygotic negative regulator of entry into mitosis (G2 to M transition) by protecting the nucleus from cytoplasmically activated cyclin B1-complexed cdk1 before the onset of mitosis by mediating phosphorylation of cdk1 on 'Tyr-15'. Specifically phosphorylates and inactivates cyclin B1-complexed cdk1 reaching a maximum during G2 phase and a minimum as cells enter M phase. Phosphorylation of cyclin B1-cdk1 occurs exclusively on 'Tyr-15' and phosphorylation of monomeric cdk1 does not occur. Involved in convergent extension of the paraxial mesoderm during neurulation by inhibiting the cell cycle. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Zygotically expressed. Expressed in regions of the embryo that are devoid of mitotic cells, such as the involuting mesoderm. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily. Was initially assigned as wee2 (PubMed:12217326). However, it corresponds to the zygotic protein WEE1 in mammals.