SNCA and Parkinson disease: However, the problem with this hypothesis concerns (i) what induces alpha-synuclein aggregation into neurotoxic oligomers inside of the dopaminergic neurons containing neuromelanin; and (ii) the case where the propagation of alpha-synuclein is involved in the degeneration of the nigrostriatal system, which means that the development and progression of the disease should be a very rapid process, contrasting with the extremely slow progression of Parkinson’s disease.