The transactivator CIITA regulates basal and interferon-induced expression of MHC class II genes. CIITA restores expression of all MHC II gene expression in mutant cells and corrects regulatory defects of those genes. Activation is via the proximal promoter. No DNA binding of in vitro translated CIITA was detected. CIITA may act in a coactivator-like fashion through protein-protein interactions by contacting factors binding to the proximal MHC class II promoter, to elements of the transcription machinery, or both. Alternatively it may activate HLA class II transcription by modifying proteins that bind to the MHC class II promoter.