Rad51-deficient vertebrate cells accumulate chromosomal breaks prior to cell death. In yeast, rad51 mutants are viable but sensitive to γ-rays due to defective repair of double-strand breaks (DSBs). In contrast, disruption of the murine RAD51 homologue is lethal, indicating an essential role of Rad51 in vertebrate cells. The study used the chicken B lymphocyte line DT40 to investigate Rad51's function. A human RAD51 transgene was expressed under a repressible promoter, and endogenous RAD51 was disrupted. Rad51-deficient cells accumulated in the G2/M phase of the cell cycle before dying, with most metaphase-arrested cells showing isochromatid-type breaks. These findings suggest that Rad51 is essential for repairing spontaneously occurring chromosome breaks in proliferating cells. Rad51 is mainly expressed in dividing cells, and its expression is induced in response to DNA damage. The study developed a genetic system in DT40 to study Rad51's essential function. Rad51-deficient DT40 cells showed increased chromosomal breaks, particularly isochromatid-type gaps and breaks, and eventually died. Chromosomal aberrations were more frequent in Rad51-deficient cells, and the distribution of aberrations suggested that DSBs occurred during DNA replication. The study also showed that Rad51-deficient cells had a higher frequency of chromosomal breaks, and the distribution of these breaks was stochastic. The findings indicate that Rad51 is essential for repairing DSBs in vertebrate cells, and its absence leads to chromosomal breaks and cell death. The study also suggests that Rad51 may be involved in DNA replication and recombinational repair. The results highlight the importance of Rad51 in maintaining chromosomal DNA during normal cell cycling.Rad51-deficient vertebrate cells accumulate chromosomal breaks prior to cell death. In yeast, rad51 mutants are viable but sensitive to γ-rays due to defective repair of double-strand breaks (DSBs). In contrast, disruption of the murine RAD51 homologue is lethal, indicating an essential role of Rad51 in vertebrate cells. The study used the chicken B lymphocyte line DT40 to investigate Rad51's function. A human RAD51 transgene was expressed under a repressible promoter, and endogenous RAD51 was disrupted. Rad51-deficient cells accumulated in the G2/M phase of the cell cycle before dying, with most metaphase-arrested cells showing isochromatid-type breaks. These findings suggest that Rad51 is essential for repairing spontaneously occurring chromosome breaks in proliferating cells. Rad51 is mainly expressed in dividing cells, and its expression is induced in response to DNA damage. The study developed a genetic system in DT40 to study Rad51's essential function. Rad51-deficient DT40 cells showed increased chromosomal breaks, particularly isochromatid-type gaps and breaks, and eventually died. Chromosomal aberrations were more frequent in Rad51-deficient cells, and the distribution of aberrations suggested that DSBs occurred during DNA replication. The study also showed that Rad51-deficient cells had a higher frequency of chromosomal breaks, and the distribution of these breaks was stochastic. The findings indicate that Rad51 is essential for repairing DSBs in vertebrate cells, and its absence leads to chromosomal breaks and cell death. The study also suggests that Rad51 may be involved in DNA replication and recombinational repair. The results highlight the importance of Rad51 in maintaining chromosomal DNA during normal cell cycling.