Although it is generally accepted that HLA class II molecules influence autoimmune disease risk by regulating the emergence, activation, and expansion of autoreactive CD4+ T lymphocytes, our knowledge of how HLA class II-mediated antigen presentation confers risk for autoimmune diseases and regulates CD4+ T cell autoreactivity at the molecular level is incompletely understood (219, 220). The gene discussed is CD4; the disease is autoimmune disease.