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An animal can produce an estimated 1(}"-10" unique antibodies'. One goal in molecular immunology is to understand the mechanisms responsible for the generation of this vast repertoire. Antibody molecules are composed of heavy (H) and light (L) polypeptide chains, each containing variable (V) and constant (C) regions. The variable region contains three hypervariable regions which form the antigen-binding site-, and the constant region determines the class of heavy chain or type of light chain. Antibodies are encoded by three unlinked sets, or families, of genes. Two families code for light chainslambda {X), containing I' >^ and ('i genes, and kappa (K) containing I'^ and Q genes; and one family codes for heavy chains -heavy (H), containing ('// and Q / genes. In mice, the lambda, kappa, and heavy chain families of genes are on chromosomes 16. 6, and 12. respectively'. Early data on the [)rotein sequences of variable and constant regions suggested that they were encoded by distinct genes, which randomly associated to produce different antibodies'. The diversity in variable regions calls for many more variable genes than constant genes.