Functional antibody genes are assembled by V-D-J joining and then diversified by somatic hypermutation. This hypermutation results from stepwise incorporation of single nucleotide substitutions into the V gene, underpinning much of antibody diversity and affinity maturation. Hypermutation is triggered by activation-induced deaminase (AID), an enzyme which catalyzes targeted deamination of deoxycytidine residues in DNA. The pathways used for processing the AID-generated U:G lesions determine the variety of base substitutions observed during somatic hypermutation. Thus, DNA replication across the uracil yields transition mutations at C:G pairs, whereas uracil excision by UNG uracil-DNA glycosylase creates abasic sites that can also yield transversions. Recognition of the U:G mismatch by MSH2/MSH6 triggers a mutagenic patch repair in which polymerase eta plays a major role and leads to mutations at A:T pairs. AID-triggered DNA deamination also underpins immunoglobulin variable (IgV) gene conversion, isotype class switching, and some oncogenic translocations in B cell tumors.