Lewy bodies (LBs) are hallmark lesions in the brains of patients with Parkinson''s disease (PD) and dementia with Lewy bodies (DLB). We raised a monoclonal antibody LB509 against purified LBs from the brains of patients with DLB that strongly immunolabled LBs, and found that alpha-synuclein is one of the major components of LBs. Thus, the deposition of alpha-synuclein, an abundant presynaptic brain protein, as fibrillary aggregates in affected neurons or glial cells, was highlighted as a hallmark lesion of a subset of neurodegenerative disorders, including PD, DLB and multiple system atrophy collectively referred to as synucleinopathies. Importantly, the identification of missense mutations in and multiplication of alpha-synuclein gene in some pedigrees of familial PD has strongly implicated alpha-synuclein in the pathogenesis of PD and other synucleinopathies. We then examined the specific posttranslational modifications that characterize and underlie the aggregation of alpha-synuclein in synucleinopathy brains by mass spectrometry and using a specific antibody, and found that serine 129 of alpha-synuclein deposited in synucleinopathy lesions is selectively and extensively phosphorylated. Furthermore we generated transgenic C. elegans overexpressing alpha-synuclein in neurons, and found that overexpression of familial PD-linked mutant form of alpha-synuclein impairs functions of dopamine neurons. These findings collectively underscore the importance of deposition of alpha-synuclein as well as its phosphorylation in the pathogenesis of alpha-synucleinopathies.