A single primary RNA transcript extending over both mu (µ) and delta (δ) loci, if spliced in the two different ways shown, may generate a mature mRNA encoding the IgM heavy chain or a different one encoding IgD. The IgD hinge region is encoded by a separate exon between Cδ1 and Cδ2 (not shown).
L: light chain gene segment; VH: variable region of heavy chain; D: diversity; JH: joining region of heavy chain; mRNA: messenger ribonucleic acid; IgM: immunoglobulin M; IgD: immunoglobulin D.