Function: 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. Tissue specificity: Ubiquitous. Disease: Defects in RFX5 are a cause of bare lymphocyte syndrome type 2 (BLS2) [MIM:209920]; also known as hereditary MHC class II deficiency or HLA class II-deficient combined immunodeficiency. BLS2 is a severe combined immunodeficiency disease with early onset. It is characterized by a profound defect in constitutive and interferon-gamma induced MHC II expression, absence of cellular and humoral T-cell response to antigen challenge, hypogammaglobulinemia and impaired antibody production. The consequence include extreme susceptibility to viral, bacterial and fungal infections. Similarity: Belongs to the RFX family. Contains 1 RFX-type winged-helix DNA-binding domain. Domain: 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. PTM: Phosphorylated.