Veliparib, also named as AT-888, is poly (ADP-ribose) polymerase (PARP) inhibitor and has demonstrated excellent in vivo efficacy in a broad spectrum of preclinical tumor models in combination with a variety of cytotoxic agents. PARP is involved in DNA repair and elevated PARP levels can result in resistance to cytotoxic chemotherapy and radiation. So, PARP inhibitors hold promise as chemotherapy and radiation sensitizers. ABT-888 is also active on microsatellite instability (MSI) cell lines harboring mutations in both MRE11 and RAD50 genes compared to microsatellite stable (MSS) cell lines (wild-type for both genes).