ADP_ribosylating enzymes catalyze the transfer of ADP_ribose from NAD+ to substrates. Bacterial toxins are cytoplasmic and catalyze the transfer of a single ADP_ribose unit to eukaryotic elongation factor 2, halting protein synthesis and killing the cell. Poly(ADP-ribose) polymerases (PARPS 1-3, VPARP, tankyrase) catalyze the addition of up to 100 ADP_ribose units from NAD+. PARPs 1 and 2 are localized in the nucleaus, bind DNA, and are activated by DNA damage. VPARP is part of the vault ribonucleoprotein complex. Tankyrases regulates telomere length in part through poy(ADP_ribosylation) of telomere repeat binding factor 1 (TRF1). Poly(ADP-ribose) polymerase catalyses the covalent attachment of ADP-ribose units from NAD+ to itself and to a limited number of other DNA binding proteins, which decreases their affinity for DNA. Poly(ADP-ribose) polymerase is a regulatory component induced by DNA damage. The carboxyl-terminal region is the most highly conserved region of the protein. Experiments have shown that a carboxyl 40 kDa fragment is still catalytically active.