Fig. 1.
Organization of the mouse immunoglobulin heavy chain locus. Mature B cells express both IgM and IgD after exiting the bone marrow and migrating to secondary lymphoid tissues. Individual constant regions (CH) span ~160 kb of chromosome 12, and consist of an intronic switch (S) region (indicated by unfilled ovals) located upstream of its matching CH (indicated by filled rectangles). Cδ is a notable exception; it is preceded by a non-canonical S region called σδ. B cells co-express of IgM and IgD B cell antigen-receptors (BCRs) by virtue of the alternative splicing of primary transcripts, and assembly with either kappa (κ) or lambda (λ) light chains. Solid arrows indicate transcription start sites, dashed lines within transcripts indicate introns, solid lines within transcripts indicate exons, red lines indicate disulfide bonds.