All immunoglobulins (Ig’s) consist of one or more basic units composed of identical pairs of heavy (H) and light (L) polypeptide chains (Figure 1). Each chain folds into several compact globular domains, connected by relatively narrower but more exposed areas. Each domain is approximately 110 amino acids in length and characterized by an intrachain disulfide bond connecting two cysteine residues approximately 60 amino acid residues apart (Figure 1). All Ig polypeptide chains can be divided into an amino-terminal portion, the variable (V) region, and a carboxylterminal portion, the constant (C) region. The V regions of both H and L chains are equivalent in size to a domain. The C regions of L chains are of similar size, whereas those of H chains are two to four times longer. Based on the degree of amino acid sequence homology, the V regions have been divided into three main groups, Vk
, Vλ, and VH. The C regions have been divided into k and λ types (L chains) as well as γ, α, µ, ∈, and δ classes (H chains) according to their antigenic and serological properties. Some of the L-chain types and H-chain classes are further divided into subtypes and subclasses, respectively. Details of Ig structure are given in earlier Chapters. A general review on this subject was given by Gally (1973). This chapter is devoted to the genetic aspects of the synthesis of Ig polypeptide chains.