Like MHC class I molecules, class II molecules are also heterodimers, but in this case consist of two homogenous peptides, an α and β chain, both of which are encoded in the MHC. The subdesignation α1, α2, etc. refers to separate domains within the HLA gene; each domain is usually encoded by a different exon within the gene, and some genes have further domains that encode leader sequences, transmembrane sequences, etc.
MHC class I molecules, similar to MHC class II molecules, are heterodimers formed by two identical peptide chains, an α and β chain, both of which are encoded by the MHC gene. The designation α1, α2, etc., refers to distinct domains within the HLA gene; each domain is typically encoded by a different exon in the gene, and some genes have additional domains that encode leader sequences or transmembrane regions.