Future studies could be developed to test this hypothesis by ascertaining how genetic polymorphisms that influence the activity of enzymes such as glutathione-S-transferase which are involved in the metabolism and detoxification of neurotoxicants interact with exposure history to influence age at onset of sporadic PD (Offen et al., 1996; Seaton et al., 1997; Shimoda-Matsubayashi et al., 1997; Menegon et al., 1998; Feldman & Ratner, 1999; Dawson & Dawson, 2003). The gene discussed is HPGDS; the disease is Parkinson disease.