HLA-A and attention deficit-hyperactivity disorder: In the ADHD and ASD studies mentioned above, the sample sizes were small (fewer than 200 ASD cases in studies implicating HLA-A and HLA-DRB1 alleles, and fewer than 400 cases in the study implicating HLA-B; for ADHD, the study implicating HLA-DRB1 included fewer than 50 cases).