The genetic origin of antibody diversity has been a subject of much thought and controversy over the past 20 years. Extensive studies of immunoglobulins, particularly at the amino acid sequence level, have provided us with a general scheme of the genetic loci which encode antibody molecules (Fig. 1). But many problems remain controversial, in particular the number of V genes and the mechanisms whereby two separate genes (V and C) encode a single, continuous polypeptide chain.