The invention provides compositions and methods to inhibit the cell cycle G2 checkpoint, in particular the DNA-damage-induced G2 checkpoint, in mammalian cells including human cells. Specifically, the invention provides compositions and methods to sensitize cells to DNA-damaging agents by abrogating the cell cycle G2 checkpoint. Compounds of the invention are used to treat proliferative disorders such as cancer. The invention provides compositions and methods for selectively sensitizing G1 checkpoint impaired cancer cells to DNA-damaging agents and treatments.