CIITA and hyperinsulinemic hypoglycemia, familial, 4: 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).