Next, we determined the impact of Mre11C54 and Mre11ATLD1 on ionizing radiation sensitivity to assess the overall ability to respond to DSBs. Both mutants showed significant defects relative to controls (P < 0.05) (Fig. 5c). Finally, to assess DSB resection capacity, we examined immunofluorescent foci induced by DNA damage that were composed of RPA, which coats single-stranded DNA immediately following resection9. Consistent with a resection defect, there was a 30% reduction of ionizing radiation–induced RPA foci in Mre11ATLD1 cells (Fig. 5d).