The immunoglobulin loci of the genetically tractable chicken B cell line DT40 provide a unique opportunity to study the cellular response to endogenously generated DNA damage in a chromosomal context. Abasic sites generated by the concerted action of Activation-Induced Deaminase (AID) and Uracil DNA Glycosylase result in both homologous recombination-dependent gene conversion and translesion synthesis-dependent point mutations. The system has provided important insights into both the early stages of AID-dependent immunoglobulin gene diversification and into the relationship between pathways of DNA damage bypass. Here we describe the assays that can be employed to monitor the rate and pattern of immunoglobulin gene diversification at the light chain locus of DT40.