Each MHC Class I and Class II gene consists of several exons and several introns.The most 5' exons in both Class I and Class II genes are the regulatory sequences; these code for the factors that up-regulate the MHC gene transcription. Further downstream, there is an exon that codes for a leader polypeptide. The role of this small polypeptide is to target the nascent MHC molecules to the endoplasmic reticulum; when the mature MHC molecules are ‘‘stuffed’’with peptides and ready to be expressed, the leader polypeptide falls off. Separate exons, located 3' to the leader and in tandem array, code for separate domains of the MHC molecule(a1, a2, and a3 in the MHC Class I and a1 and a2 or b1 and b2 in the MHC Class II molecule). Finally, the exons that code for the MHC molecule’s transmembrane and cytoplasmic domains are also separate and located the most 3' in an individual gene.
