SNCA and Parkinson disease: First, Abs directed to abnormal α-synuclein, applied in a systematic manner to postmortem archived brain sections from PD, led to the identification of a staging system that followed a common progression for α-synuclein lesions.39 Together with familial genetic studies,40 as well as genome-wide association studies,12 there is little uncertainty that α-synuclein is the single most important genetic and pathological factor in PD progression and susceptibility.