If toxicant exposure contributes to the cause of Parkinson's disease, poor function of detoxifying enzymes could increase vulnerability for Parkinson's disease. Although no hepatic enzyme system has been shown universally to be dysfunctional in Parkinson's disease patients, several have been suggested to be dysfunctional in subgroups, such as those with young age at disease onset. Specific enzymes implicated include several P450 enzymes, most notably P450 IID6, and cysteine dioxygenase. If hepatic enzyme abnormalities contribute to the development of Parkinson's disease, molecular genetic tech-niques may allow the development of screening testa to identify at-risk subjects in order to intervene with protective therapies.