Schematic representation of the NLRC5 (‘CITA’) enhanceosome. NLRC5 activates MHC class I genes by generating a nucleoprotein complex with other transcription factors bound to the conserved W/S-X-Y module found in the proximal promoter region of MHC class I genes. The trimeric RFX factor complex and the transcription factor ATF1/CREB are components of the CITA enhanceosome and bind to the X1 and X2 box cis-regulatory elements, respectively. The CITA enhanceosome requires the W/S motif, yet the factor binding to the W/S motif remains unknown. Additional cis-regulatory elements (Enhancer A, ISRE) in the MHC class I promoter also modulate the activity of MHC class I promoters. 5, RFX5; ANK, RFXANK/B; AP, RFXAP; Y, NF-Y; HAT, histone acetyltransferase.