Genetically, MHC II deficiency is heterogeneous, with pathogenic variants not in HLA structural genes but in four transcriptional regulators: class II transactivator CIITA (group A) and the RFX complex subunits RFXANK (group B), RFX5 (group C), and RFXAP (group D) (3). This evidence concerns the gene RFX1 and hyperinsulinemic hypoglycemia, familial, 4.