A functional immune system depends on the production of a wide variety of immunoglobulin molecules. Immunoglobulin variable region (IgV) genes are diversified after gene rearrangement by a process termed ‘hypermutation’. In the current DNA deamination model, deamination of dC residues to dU by activation-induced deaminase (AID) is processed to trigger this diversification. The excision activity of uracil DNA glycosylase removes these dU residues, generating AP sites. In hypermutating chicken DT40 B cells, most IgV mutations are dC to dG/dA, or dG to dC/dT transversions, which are proposed to result from replication over these AP sites. While DNA glycosylases generally repair base adducts and thereby prevent mutation, impairing their function is essential for a diversified immune system. Thus, a base excision repair enzyme plays an important catalytic role in antibody diversification following dC to dU deamination events within the immunoglobulin locus itself.