Activates transcription from class II MHC promoters. Recognizes X-boxes. Mediates cooperative binding between RFX and NF-Y. RFX binds the X1 box of MHC-II promoters. Homodimer. The RFX heterotetrameric complex consists of 2 molecules of RFX5 and one each of RFXAP and RFX-B/RFXANK; with each subunit representing a separate complementation group (PubMed:10779326). Interacts (via PxLPxI/L motif) with RFXANK (via ankyrin repeats); the interaction is direct (By similarity). RFX forms cooperative DNA binding complexes with X2BP and CBF/NF-Y. RFX associates with CIITA to form an active transcriptional complex (PubMed:10779326). The N-terminus is required for dimer formation, association with RFXANK and RFXAP, assembly of the RFX complex, and for binding of this complex to its X box target site in the MHC-II promoter. The C-terminus mediates cooperative binding between the RFX complex and NF-Y. The PxLPxI/L motif mediates interaction with ankyrin repeats of RFXANK. Phosphorylated. Belongs to the RFX family.