CIITA and MHC class II deficiency: Previous studies could show that the control of de novo MHC class II expression in various cell types is tightly regulated by several key factors (reviewed in (24)), namely, the MHC class II transactivator CIITA (49, 50), CREB1 (43), NFY (51), and the RFX complex (composed of RFX5 (52), RFXAP (53), and RFXANK (54)), and that genetic deficiencies in those key factors can cause severe pathologies such bare lymphocyte syndrome (BLS) (49) and lymphoid cancers (69).