The poly(ADP-ribose) polymerases (PARPs) form a family of enzymes with roles in DNA repair and apoptosis, particularly in response to reactive oxygen and nitrogen species. 4-amino-1,8-Naphthalimide (4-ANI) is an inhibitor of PARP (IC = 180 nM). It blocks radiation-induced PARP in cancer cells, potentiating the cytotoxicity of γ-radiation, although it is not cytotoxic in the absence of radiation. 4-ANI is used to study the role of PARP activity in various cell systems.