Poly (ADP-ribose ) Polymerase (PARP) is a kind of enzyme which responsible for posttranslation modification of proteins. It exists in many eukaryotic cells and catalyzes poly ADP-ribosylation of nuclear proteins including histone H₁ and itself. When cells are damaged by toxic agents that induce DNA breakages, PARP molecules will bind to DNA breakage and their catalytic activity is activated. Then they modify their acceptor protein and trigger a set of cascade reactions. Thus PARP is a possible molecular sensor and transducer to initiate the signal transferring path for reacting to damages of the cell. The information which they passed may decide the fate of a cell: repair or death.