Acts as a negative regulator of entry into mitosis (G2 to M transition) by phosphorylation of Cdk1 specifically when Cdk1 is complexed to cyclins (PubMed:11882391, PubMed:12072468, PubMed:15581871). Mediates phosphorylation of Cdk1 predominantly on 'Thr-14' (PubMed:11882391). Also involved in Golgi fragmentation (PubMed:11882391). May be involved in phosphorylation of Cdk1 on 'Tyr-15' to a lesser degree, however tyrosine kinase activity is unclear and may be indirect (PubMed:11882391). May be a downstream target of Notch signaling pathway during eye development (PubMed:12072468). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily. Truncated N-terminus.