Figure 1: Poly(ADP-ribosyl)ation. PARPs cleave the glycosidic bond of NAD+ between nicotinamide and ribose followed by the covalent modification of acceptor proteins with an ADP-ribosyl unit. PARPs also catalyze an adduct elongation, giving rise to linear polymers with chain lengths of up to 200 ADP-ribosyl units, characterized by their unique ribose (1′′→2′) ribose phosphate-phosphate backbone. At least some of the PARP family members also catalyze a branching reaction by creating ribose (1′′′→2′′) ribose linkages.