DT40 is a chicken B lymphocyte cell line that exhibits a high ratio of targeted and random integration of transfected DNA constructs. Using the DT40 cell line makes it comparatively easy to disrupt multiple genes in a single cell and to generate conditional targeted mutants including tet-controlled cre-lox-mediated and temperature-sensitive mutants. The DT40 mutants show a strong phenotypic resemblance to murine mutants with respect to genes involved in DNA recombination and repair. Because of these characteristics, DT40 is an attractive model for the analysis of DNA recombination and repair studies in vertebrates despite obvious concerns associated with the use of a transformed cell line that may have certain cell-line-specific characteristics. We present experimental evidence to demonstrate the usefulness of the DT40 cell line as a unique model to study DNA damaging events and their associated repair pathways.