(a) The rearranged V(D)J exon is located immediately upstream of the μ constant region with all other classes of murine constant region genes lying downstream, with each (except Cδ) preceded by repetitive DNA sequences (termed switch (S) regions) that are between 1-12 kb in length. (b) The variable region exon or S regions targeted for modification are rendered accessible by transcription. Secondary structures, shown here for S regions, are formed and allow for direct SS DNA modification by AID, resulting in dU/dG mismatches. Differential sensing, processing, and resolution of these mismatches results in distinct outcomes for CSR or SHM.