PARP (Poly (ADP-ribose) polymerase) is a 113 kD nuclear protein that can exist as a homo- or hetero-dimer. This protein acts as a molecular "nick sensor" and functions in base excision repair, poly(ADP-ribosyl)ation of acceptor proteins involved in chromatin architecture and DNA metabolism and participates in protein modification to enhance or repress transcription. PARP is ribosylated by PARP2 and is a target for caspase cleavage during apoptosis. PARP interacts with proteins in the base excision repair complex containing at least XRCC1, PARP2, POLB and LIG3. In addition PARP forms heterodimers with PARP2, and interacts with PARP3. The 5A5 monoclonal antibody recognizes the N-terminal region of human and mouse PARP and has been shown to be useful for Western blotting and immunofluorescence staining.