Protein kinase that acts both on serines and on tyrosines. It acts as a negative regulator of entry into mitosis (G2 to M transition). Phosphorylates and inhibits cdc2. 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.