Small seeds of fibrillar α-synuclein corrupt endogenous α-synuclein to form inclusions biochemically and morphologic resembling those found in PD and DLB (Volpicelli-Daley et al., 2011; Luk et al., 2012; Paumier et al., 2015; Froula et al., 2019; Sorrentino et al., 2019). Here, SNCA is linked to Parkinson disease.