Taking into account the familial cases, i.e., mutations in SNCA, LRRK2, GBA, UCHL1, VPS35, PRKN, PINK1, ATP13A2, PLA2G6, DNAJC6, SYNJ1, DJ-1/PARK7 and FBXO7 genes, it appears that (i) mutant forms of α-syn can aggregate and lead to early-onset PD and (ii) that mutations in other genes lead to deposition of non-mutated α-syn and early onset of the EP. This evidence concerns the gene SNCA and Parkinson disease.