Major histocompatibility complex class II (MHC II) deficiency, also known as bare lymphocyte syndrome type II, is an autosomal recessive combined immunodeficiency caused by mutations in one of four key transcriptional regulators of MHC-II gene expression: CIITA (group A), RFXANK (group B), RFX5 (group C) or RFXAP (group D) (1–3). This evidence concerns the gene RFX5 and hyperinsulinemic hypoglycemia, familial, 4.