A family of trans-acting factors with distinct regulatory functions control expression of MHC class II genes.

The regulation of expression of MHC class II genes is mainly exerted at the transcription level. Three short and conserved sequences have been identified at the gene promoter region, common to all MHC class II genes, named W, X and Y, where specific trans-acting factors bind, to initiate transcription. We have recently identified a trans-acting factor encoded by the Activator Immune Response genes-1 (AIR-1) locus, whose action is essential to guarantee MHC class II gene transcription, apparently acting on all three regions, although the binding features are still to be defined.