HLA-DRB1 and Alzheimer disease: Last, the HLA-amino acid–level analysis emphasized the pair of HLA-DRB1 amino acids H13 and H33 as the most significant amino acid changes associated with AD and PD risks (H13 association with AD: OR = 0.91[0.89; 0.93]; P = 1.7E-18; and with PD: OR = 0.89[0.87; 0.91]; P = 9.3E-22).