Targeting of AID to antibody variable (V) regions results in somatic hypermutation, whereas its recruitment to switch (S) regions leads to class-switch recombination. Yeap et al. find that the mechanism by which variable and switch regions recruit AID essentially is the same but that the two regions differ in the density of double-stranded DNA breaks that are generated. These lead to either point mutations in V exons in somatic hypermutation or deletion of intervening DNA sequences during class switch recombination.
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