Although V(D)J recombination is subefficient in mice in vivo, DSBs are ultimately repaired since Xlf−/−Trp53−/− DKO do not develop T or pro-B cell lymphomas (22) and the immune phenotype is even rescued on a Trp53−/− background (24). The gene discussed is NHEJ1; the disease is B-cell non-Hodgkin lymphoma.