Pillars article: AID mutates E. coli suggesting a DNA deamination mechanism for antibody diversification. Nature. 2002. 418: 99-103

After gene rearrangement, immunoglobulin variable genes are diversified by somatic hypermutation or gene conversion, whereas the constant region is altered by class-switch recombination. All three processes depend on activation-induced cytidine deaminase (AID)^1-7, B-cell-specific protein that has been proposed (because of sequence homology¹) to function by RNA editing. But indications that the three gene diversification processes might be initiated by a common type of DNA lesions^8-11, together with the proposal that there is a first phase of hyper-mutation that targets dC/dG¹², suggested to us that AID may function directly at dC/dG pairs. Here we show that expression of AID in Escherichia coli gives a mutator phenotype that yields nucleotide transitions at dC/dG in a context-dependent manner. Mutation triggered by AID is enhanced by a deficiency of uracil-DNA glycosylase, which indicates that AID functions by deaminating dC residues in DNA. We propose that diversification of functional immunoglobulin genes is triggered by AID-mediated deamination of dC residues in the immunoglobulin locus with the outcome - that is, hypermutation phases 1 and 2, gene conversion or switch recombination - dependent on the way in which the initiating dU/dG lesion is resolved.