The RFX complex is crucial for the constitutive and CIITA-mediated transactivation of MHC class I and beta2-microglobulin genes.

In type III bare lymphocyte syndrome (BLS) patients, defects in the RFX protein complex result in a lack of MHC class II and reduced MHC class I cell surface expression. Using type III BLS cell lines, we demonstrate that the RFX subunits RFX5 and RFXAP are crucial for constitutive and CIITA-induced MHC class I and beta2m transactivation. Similar to MHC class II, the promoters of MHC class I and beta2m contain an S-X-Y region of which the X1 box is crucial for constitutive and CIITA-induced MHC class I and beta2m transactivation. Thus, the RFX complex is part of a regulatory pathway linking the transactivation of MHC class I and II and their accessory genes.