An Error has occurred retrieving Wikidata item for infobox MHC class II transactivator is a major histocompatability complex protein encoded in the human by the gene CIITA. Mutations in this gene are responsible for the bare lymphocyte syndrome in which the immune system is severely compromised and cannot effectively fight infection. Chromosomal rearrangement of CIITA is involved in the pathogenesis of Hodgkin lymphoma and primary mediastinal B cell lymphoma.