HLA-DRB1 and attention deficit-hyperactivity disorder: Among the genes implicated in the study, which included both quantitative and case–control analyses, were HLA-A (major histocompatibility complex, class I, A), HLA-B (major histocompatibility complex, class I, B) and HLA-DRB1 (major histocompatibility complex, class II, DR beta 1), which were previously implicated in autism [43], attention-deficit/hyperactivity disorder (ADHD) [44, 45] and schizophrenia [46].