ERCC2 and xeroderma pigmentosum: The relationship between genetic variation and environmental influence on disease risk has also been demonstrated between two non-synonymous xeroderma pigmentosum D (XPD) polymorphisms and sun exposure on skin cancer (87), and between a XPD polymorphism and the risk of breast cancer, especially from polycyclic aromatic hydrocarbon (PAH)-DNA adducts found in cigarette smoke (88).