Loss-of-function mutations of the parkin gene are a major cause of early-onset parkinsonism. To explore the mechanism by which loss of parkin function results in neurodegeneration, we are using a genetic approach in Drosophila. Here, we show that Drosophila parkin mutants display degeneration of a subset of dopaminergic (DA) neurons in the brain. The neurodegenerative phenotype of parkin mutants is enhanced by loss-of-function mutations of the glutathione S-transferase S1 (GstS1) gene, which were identified in an unbiased genetic screen for genes that modify parkin phenotypes. Furthermore, overexpression of GstS1 in DA neurons suppresses neurodegeneration in parkin mutants. Given the previous evidence for altered glutathione metabolism and oxidative stress in sporadic Parkinson's disease (PD), these data suggest that the mechanism of DA neuron loss in Drosophila parkin mutants is similar to the mechanisms underlying sporadic PD. Moreover, these findings identify a potential therapeutic approach in treating PD.genetic modifier ͉ neurodegeneration ͉ parkin P arkinson's disease (PD) is a common neurodegenerative disorder characterized by the loss of dopaminergic (DA) neurons in the substantia nigra and the accumulation of proteinaceous intraneuronal inclusions known as Lewy bodies. The mechanisms responsible for neurodegeneration in PD are largely unknown, although previous work suggests that mitochondrial complex I dysfunction, oxidative stress, and aberrant proteolytic degradation may contribute to pathogenesis (1). The recent identification of genes responsible for rare inherited forms of parkinsonism presents an opportunity to establish neurodegenerative mechanisms that may be relevant to sporadic forms of PD.Loss-of-function mutations of parkin are a common cause of autosomal recessive juvenile parkinsonism (ARJP), and parkin dysfunction may also contribute to late-onset sporadic PD (2-6). Patients with parkin mutations display many of the typical features of idiopathic PD, including locomotor dysfunction, reduced mitochondrial complex I activity, and degeneration of DA neurons in the substantia nigra. However, most ARJP cases have a significantly earlier age of onset and lack Lewy body pathology. Parkin has been shown to possess ubiquitin-protein ligase activity (7-9), which acts to confer substrate target specificity in the ubiquitin͞proteasome protein degradation pathway. This finding has led to the model that toxic accumulation of parkin substrates may be responsible for DA neuron death. A number of putative substrates of parkin have been identified (10). Several of these parkin substrates, including the Lewy body component ␣-synuclein (11) and the putative G protein-coupled receptor Pael-R (12), have received considerable attention, in part because they implicate specific cellular pathways in parkinmediated pathogenesis. However, the involvement of many of the identified parkin substrates in the etiology of ARJP remains controversial.To identify pathways relevant to parkin pathogenesis, we are using a gen...