CDK2 activity drives a cell into, and through, S-phase of the cell cycle where the genome is duplicated in preparation for cell division. Inhibition of Wee1 is expected to cause aberrantly high CDK2 activity in S-phase cells which, in turn, leads to unstable DNA replication structures and ultimately DNA damage. Therefore, it is anticipated that AZD1775 will have independent anti-tumor activity in the absence of added chemotherapy.