Figure 2.

Schematics representing class switch recombination from Sμ to Sα. Recombining switch domains are rendered accessible to activation-induced cytidine deiminase (AID) and replication protein A (RPA) by the concerted action of I-promoters and the μ (5′) and α (3′) enhancers (E). Removal of uracils by uracil-N-glycosylase 1 (UNG) and nicking of the DNA backbone by Ape nucleases results in the formation of staggered DNA breaks, which are either repaired in an error-free manner by base excision repair (BER) or serve as substrates for double-strand break (DSB) repair and/or recombination. Although not depicted, mismatch repair activity induces class switch recombination via a similar mechanism.