Wee1 is a nuclear protein involved in the regulation of the G2/M checkpoint by negatively regulating entry into mitosis by catalyzing an inhibitory tyrosine phosphorylation of the Cdc2/cyclin B kinase complex.60 Activation of this complex (also known as maturation-promoting factor or MPF) is necessary to allow the entry into mitosis as it is involved in the direct modulation of the functions of number of proteins during cell division. Importantly, MPF only becomes active once DNA replication has been completed in order to permit a normal transition through the cell cycle. There are several levels of regulation of MPF activity, but Wee1 tyrosine kinase activity is a major regulatory component.

