RFXAP and combined immunodeficiency: HLA class II deficiency is an AR CID marked by the absence or greatly reduced expression of major histocompatibility class II molecules on the cell surface due to mutations in the MHC class II transactivator (CIITA), regulatory factor X-associated protein (RFXAP), regulatory factor X-5 (RFX5), and ankyrin-repeat-containing regulatory factor X (RFXANK) [295].