Abstract Defects in Brca1 confer susceptibility to breast cancer and genomic instability indicative of aberrant repair of DNA breaks. Brca1 was previously implicated in the homologous recombination pathway via effects on the assembly of recombinase Rad51. Activation-induced cytidine deaminase (AID) deaminates C to U in B lymphocyte immunoglobulin ( Ig) DNA to initiate programmed DNA breaks. Subsequent uracil-glycosylase mediated U removal, and perhaps further processing, leads to four known classes of mutation: Ig class switch recombination that results in a region-specific genomic deletion, Ig somatic hypermutation that introduces point mutations in Ig V-regions, Ig gene conversion in vertebrates that possess Ig pseudo- V genes, and translocations common to B cell lymphomas. We tested the involvement of Brca1 in AID-dependent Ig diversification in chicken DT40 cells. The DT40 cell line diversifies Ig Vλ mainly by gene conversion, and less so by point mutation. Brca1-deficiency caused a shift in Vλ diversification, significantly reducing the proportion of gene conversions relative to point mutations. Thus, Brca1 regulates AID-dependent DNA lesion repair. Interestingly, while Brca1 is required to recruit ubiquitinated FancD2 to DNA damage, the phenotype of Brca1-deficient DT40 differs from the one of FancD2-deficient DT40, in which both gene conversion and non-templated mutations are impaired.