In this study, GYY4137 (100 μM to 1 mM) caused concentration-dependent G2/M phase cell cycle arrest and PARP-/caspase-9-dependent apoptosis of seven different human cancer cell lines (HeLa, HCT-116, HepG2, HL-60, MCF-7, MV4-11 and U2OS) but did not affect normal human lung fibroblasts (IMR90, WI-38) suggesting that cancer cells can be killed selectively. This evidence concerns the gene CASP9 and cancer.