At least 90 genetic loci have been associated with PD risk in genome-wide association studies (GWAS), including the HLA-DRA, -DRB, and -DQ genes within the Major Histocompatibility Complex (MHC) class II region on the short arm of chromosome 6 at 6p21.3 (27, 28). Here, HLA-DRA is linked to Parkinson disease.