HLA-DRB1 and Alzheimer disease: In addition to the previously mentioned proteins, the major histocompatibility complex, class I, C (HLA-C), the major histocompatibility complex, class II, DQ alpha 1 (HLA-DQA1), and the major histocompatibility complex, class II, DR beta 1 (HLA-DRB1) have been linked to AD, Parkinson’s disease, and multiple sclerosis.