Regulatory factor essential for MHC class II genes expression. Binds to the X boxes of MHC class II genes. Also binds to an inverted repeat (ENH1) required for hepatitis B virus genes expression and to the most upstream element (alpha) of the RPL30 promoter. Homodimer; binds DNA as a homodimer (PubMed:10706293). Heterodimer; heterodimerizes with RFX2 and RFX3 (PubMed:10330134). Belongs to the RFX family.