Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein kinases in the fission yeast Schizosaccharomyces pombe (S. pombe). Wee1 has amolecular mass of 96 kDa and it is a key regulator of cell cycle progression. Wee1 influences cell size by inhibiting the entry into mitosis, through inhibiting Cdk1. Wee1 has homologues in many other organisms, including mammals. Wee1 inhibits Cdk1 by phosphorylating it on two different sites, Tyr15 and Thr14. Cdk1 is crucial for the cyclin-dependent passage of the various cell cycle checkpoints. At least three checkpoints exist for which the inhibition of Cdk1 by Wee1 is important: G2/M checkpoint, Cell size checkpoint, DNA damage checkpoint. Wee1 is shown to phosphorylate histone H2B at tyrosine 37 residue which regulates global expression of histones.