Over the last few years, several genes for monogenically inherited forms of Parkinson's disease have been mapped and/or cloned. In a large family with autosomal dominant inheritance and typical Lewy-body pathology, a first gene locus has been mapped to the long arm of chromosome 4, and mutations in this and a few other families linked to this locus have been identified in the gene for alpha-synuclein. Aggregation of this protein in Lewy bodies may be a crucial step in the molecular pathogenesis of familial and sporadic Parkinson's disease. A gene causing autosomal recessive parkinsonism of juvenile onset has been mapped to chromosome 6, and the causative gene has been identified and named parkin. A third locus, again in families with dominant inheritance, typical Lewy-body pathology, and late onset, has been mapped to chromosome 2p13, and two additional genes on chromosome 4p have been linked to other dominantly inherited forms of the disease. At present, there is no direct evidence that any of the genes for familial parkinsonian syndromes has a direct role in the etiology of the common sporadic form of PD. However, the elucidation of the molecular sequence of events leading to nigral degeneration in these inherited cases is likely also to shed light on the molecular pathogenesis of the common sporadic disorder.