Activation-Induced Cytidine Deaminase Links Class Switch Recombination and Somatic Hypermutation

: Activation‐induced cytidine deaminase (AID), a putative RNA‐editing cytidine deaiminase that is expressed strictly in activated B cells, is indispensable in three apparently distinct genetic alterations of immunoglobulin genes—namely, class switch recombination, somatic hypermutation, and gene conversion. Recent findings led us to propose a common DNA cleaving mechanism, in which the transient secondary structure of the S and V region DNA is recognized by a nicking enzyme regulated by the putative RNA‐editing activity of AID.