The formation of point mutations in the variable region of antibody genes, templated insertion of DNA sequence (gene conversion), and immunoglobulin class switching are all outcomes of a single basic process. AID catalyzes the deamination of cytosine to uracil, followed by removal of the uracil base by uracil-DNA glycosylase and repair bymismatch-repair enzymes and DNA polymerase η, homologous recombination with a nonfunctional V gene segment (V-pseudogene), or class-switch recombination mediated by DNA-dependent protein kinase (DNA-PK) and KU70/80. AP, apyrimidinic; dRPase, deoxyribophosphodiesterase; MSH2, mutS homolog 2, mismatch repair enzyme; MSH6, mutS homolog 6, mismatch repair enzyme; RAD51B, homologous recombination repair protein; XRCC2, X-ray repair cross-complementing group 2, homologous recombination repair protein; XRCC3, X-ray repair cross-complementing group 3, homologous recombination repair protein. Figure reproduced with permission from Nature.