The study investigates the role of Rad51 in vertebrate cells, focusing on the chicken B lymphocyte line DT40. Rad51-deficient cells accumulate chromosomal breaks before dying, accumulating in the G2/M phase of the cell cycle. Chromosome analysis reveals that most metaphase-arrested Rad51+ cells carry isochromatid-type breaks. The results suggest that Rad51 is essential for the repair of spontaneous chromosome breaks in proliferating cells of higher eukaryotes. The study also discusses the potential mechanisms by which Rad51 deficiency leads to chromosomal breakage and the significance of spontaneous chromosomal breaks in cycling vertebrate cells.The study investigates the role of Rad51 in vertebrate cells, focusing on the chicken B lymphocyte line DT40. Rad51-deficient cells accumulate chromosomal breaks before dying, accumulating in the G2/M phase of the cell cycle. Chromosome analysis reveals that most metaphase-arrested Rad51+ cells carry isochromatid-type breaks. The results suggest that Rad51 is essential for the repair of spontaneous chromosome breaks in proliferating cells of higher eukaryotes. The study also discusses the potential mechanisms by which Rad51 deficiency leads to chromosomal breakage and the significance of spontaneous chromosomal breaks in cycling vertebrate cells.