sentences:  The beta chain is approximately 26-28 kDa.It is encoded by 6 exons, exon one encodes the leader peptide, exons 2 and 3 encode the two extracellular domains, exon 4 encodes the transmembrane domain and exon 5 encodes the cytoplasmic tail.Within the DR molecule the beta chain contains all the polymorphisms specifying the peptide binding specificities.