HLA-DRB1 and Parkinson disease: Conversely, the HLA-DRB1*0406 was found with lower allele frequency in PD patients compared to healthy controls (0.0013 vs. 0.0308, p = 8.036E-7 in series 1, 0.0078 vs. 0.0267, p = 0.097 in series 2, 0.0035 vs. 0.0295, p = 3.229E-6 in combined series 1 and series 2).