AZD1775 is a highly selective, adenosine-triphosphate (ATP) competitive, small-molecule inhibitor of Wee1 kinase. Wee1 is a tyrosine kinase involved in regulation of intra-S and G2 cell cycle checkpoints through phosphorylation and inhibition of CDK2 and CDK1, respectively. Because activity of these and other CDKs coordinate progression through the cell cycle, they are inhibited at cell cycle checkpoints, causing transient arrest at the G1-, S-, and G2 phases of the cell cycle.