The WGR domain is found in a variety of eukaryotic poly(ADP-ribose) polymerases (PARPs). It has been called WGR after the most conserved central motif of the domain. The domain typically occurs together with a catalytic PARP domain, and is between 70 and 80 residues in length. It has been proposed to function as a nucleic acid binding domain. PARPs catalyze the NAD(+)-dependent synthesis of ADP-ribose polymers and their addition to various nuclear proteins and histones. Higher eukaryotes contain several PARPs and there may be up to 17 human PARP-like proteins, with three of them (PARP-1, PARP-2, and PARP-3) containing a WGR domain. The synthesis of poly-ADP-ribose requires multiple enzymatic activities for initiation, trans-ADP-ribosylation, elongation, branching, and release of the polymer from the enzyme. Poly-ADP-ribosylation was thought to be a reversible post-translational covalent modification that serves as a regulatory mechanism for protein substrates. However, it is now known that it plays important roles in many cellular processes including maintenance of genomic stability, transcriptional regulation, energy metabolism, cell death and survival, among others.