IgH rearrangements (VH-D, D-JH) are central to the generation of antibody diversity. The majority of the diversity seen in the third hypervariable region is generated by the D segment and at the joints formed by both junctional and N segment variation during D-JH and VH-D rearrangements. The mechanisms that regulate rearrangement are thought to obey the 12/23 rule, wherein D-D or VH-JH rearrangements are precluded. Here, we present evidence that D-D fusions do in fact occur, either as direct or inverted rearrangements. The fused D segments so generated may be fully capable of proceeding in subsequent D-JH and VH-D rearrangements. The resultant VH-D-D-JH recombinations add another dimension to the potential repertoire of IgH V regions by increasing the level of combinatorial diversity and by providing additional sites for N region variation.

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