RFXAP (UniProtKB/Swiss-Prot entry O00287) is part of the RFX complex that binds to the X-box of MHC II promoters. The RFX complex consists of at least 3 different subunits; RFXAP, RFX5 and RFX-B/RFXANK; with each subunit representing a separate complementation group. RFX forms cooperative DNA binding complexes with X2BP and CBF/NF-Y. RFX associates with CIITA to form an active transcriptional complex. Defects in RFXAP can cause bare lymphocyte syndrome type II (BLS II); also known as hereditary MHC class II deficiency or HLA class II-deficient combined immunodeficiency.