Ig class switching: targeting the recombinational mechanism.

Recent studies have provided insights into the mechanisms involved in targeting antibody gene class switch recombination (CSR) to various switch DNA regions located upstream of constant region genes. Targeting appears to involve sequence motifs that are favored for deoxycytosine deamination by the activation-induced deaminase enzyme that is required for CSR, together with transcription (and in some cases R-loop formation) to provide the single-stranded DNA needed for activation-induced deaminase activity. There is also another poorly understood mechanism that limits CSR to a specific length of DNA downstream of the switch-region transcriptional promoter.