Therefore, this literature review will assume that the cell-to-cell transmission of pathological alpha-synuclein in PD is dependent upon SNCA gene transcription, alpha-synuclein production, alpha-synuclein aggregation and alpha-synuclein uptake into neighboring neurons, in addition to insufficient alpha-synuclein intracellular and extracellular degradation processes. This evidence concerns the gene SNCA and Parkinson disease.