Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans(PCDFs), and dioxin-like polychlorinated biphenyls (dl-PCBs) represent aclass of toxic semi-volatile aromatic compounds commonly referred to as being 'dioxin-like'. The dioxin-like classification combines these organic compounds into a single chemical class defined as having analogous chemical and physical properties, chlorine substitution patterns, a planar molecular orientation, a common mode of action of toxicity in mammals, and common endpoints or manifestations of toxicity.The compound 2,3,7,8-tetrachlor odibenzo-p-dioxin (TCDD) is the prototypedioxin-like compound and serves as the reference compound to this class.