Fig. 5. Immunoglobulin gene rearrangements in the production of γ1 heavy chain. In the germline DNA, there are approximately 200 variable region genes (VH), nine diversity genes (D), and six joining region genes (J). There also are nine constant region genes (C). The first rearrangement of the heavy-chain genes results in an active-gene complex that contains only one V, one D, and one J, the other VDJs having been deleted. When class-switching occurs to γ1, the constant region genes between the VDJ and C γ1 are deleted (i.e., Cμ, Cδ, and Cγ3). The γ1 heavy chain is now ready to be transcribed to synthesize mRNA for this particular γ1 heavy chain. (After Medical Knowledge Self-Assessment Program [MKSAP]: Allergy and Immunology, Book 1, p 157. Philadelphia: American College of Physicians, 1993.)