sentences:  It is present in the first locus (coding for C4A) for all C4 genes and in the second locus (coding for C4B) only in a few C4 genes.The α-chain consists of residues 661-1428, encoding exons 16-33.Within this chain, two cleavage sites marked by exons 23 and 30 produces the C4d fragment (where the thioester, Ch/Rg antigens, and isotypic residues are located); moreover, most of the polymorphic sites cluster in this region.