Mutations of the Regulatory Factor X5 (<i>RFX5</i>) have been associated with the autosomal recessive major histocompatibility class II (MHC-II) deficiency, which is a severe immunodeficiency characterized by constitutive and interferon-gamma induced MHC II expression disorder and leads to the absence of cellular and humoral T-cell response to antigen challenge. The gene discussed is RFX5; the disease is hyperinsulinemic hypoglycemia, familial, 4.