Poly(ADP-ribose) polymerases catalyse 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. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes pleotropic effects in mammalian species through modulating gene expression. TCCD indicible PARP (TiPARP) is a target of TCDD that may contribute to multiple responses to TCDD by modulating protein function through poly ADP-ribosylation