One B-lymphocyte produces–and displays on its surface–one, and only one antibody, one immunoglobulin (Ig) structure in 104 to 105 copies with one heavy (H) and one light (L) chain amino acid sequence. This central dogma of immunology, the basis of the clonal selection hypothesis (Jerne, 1955, Burnet, 1957) is observed in mouse B cell development by the H chain locus for 98% to 99%, and with the L chains loci for 95% to 97, 5% of all peripheral splenic and lymph node B cells.(For a review see9) Hence, in the vast majority of these B cells–but not necessarily in other B cell lineages such as the BI lineage–only one of two IgH-, and only one of six IgL (one L and three L chain loci) alleles are translated into proteins which assemble to Ig molecules and are deposited as antigen-specific receptors (BcR’s) on the cell surface. This choice of one allele of the H-chain and L chain loci is termed “allelic exclusion”.

The molecular mechanisms and cellular developmental pathways, by which B-lymphocyte lineage cells achieve this allelic exclusion, appear to be different for the H-and the L chain loci. Ig gene loci have to undergo V (D) J-recombination before they can be expressed as mature mRNA molecules and translated into protein.(For reviews see 8, 9)