Poly [ADP-ribose] polymerase 1 (PARP-1) also known as NAD+ ADP-ribosyltransferase 1 or poly [ADP-ribose] synthase 1 is an enzyme that in humans is encoded by the PARP1 gene. It is the most abundant of the PARP family of enzymes, accounting for 90% of the NAD+ used by the family. PARP1 is mostly present in cell nucleus, but cytosolic fraction of this protein was also reported. PARP1 acts as a first responder that detects DNA damage and then facilitates choice of repair pathway. PARP1 contributes to repair efficiency by ADP-ribosylation of histones leading to decompaction of chromatin structure, and by interacting with and modifying multiple DNA repair factors. PARP1 is implicated in the regulation of several DNA repair processes including the pathways of nucleotide excision repair, non-homologous end joining, microhomology-mediated end joining, homologous recombinational repair, and DNA mismatch repair.