Poly(ADP-ribose) polymerase (PARP), a nuclear enzyme often termed a molecular nick sensor, recognizes and binds to DNA single or double strand breaks through the N-terminal DNA binding domain sensor. The activation of PARP is one of the first responses to DNA strand interruptions. The DNA-bound, catalytically activated PARP synthesizes poly(ADP-ribose)(pADPr) on nuclear target proteins such as topoisomerase, histones, and PARP itself. Although the exact role of PARP is not well known, it has been implicated in different cellular responses to genotoxic damage. These responses include cell survival and death, transformation, and DNA repair. Additional studies have demonstrated the role of PARP in the death of neuronal cells after ischemia-reperfusion injury, as well as maintaining the genomic integrity and survival responses of PARP knockout mice after exposure to whole body gamma-irradiation.