The CIITA (Class II Major Histocompatibility Complex Transactivator) gene acts as the "master control switch" for the adaptive immune system. It does not bind DNA directly but functions as a transcriptional coactivator, encoding the CIITA protein, which assembles a multi-protein complex (the enhanceosome) to initiate the expression of MHC class II genes. While constitutively expressed in professional antigen-presenting cells like B cells and dendritic cells, its expression can be induced by interferon-gamma (IFN-γ) in almost any nucleated cell type, such as epithelial or endothelial cells ^[1]. Mutations in CIITA lead to Bare Lymphocyte Syndrome Type II (BLS II), a severe combined immunodeficiency characterized by the absence of MHC II molecules, resulting in a failure of CD4+ T cell development and a compromised immune response ^[2].

Ciita-iCre mice were generated using gene editing technology to replace the TGA stop codon with the P2A-iCre cassette. Under the control of the Ciita gene's regulatory elements, the iCre recombinase (a codon-optimized Cre recombinase) is expressed. When Ciita-iCre mice are crossed with mice containing loxP sites, Cre recombinase-mediated recombination between loxP sites is expected to occur in the antigen-presenting cells (APCs) and Ciita-positive cells in the offspring.