53BP1 Inhibits Homologous Recombination in Brca1-Deficient Cells by Blocking Resection of DNA Breaks Defective homologous recombination (HR) is a major contributor to tumorigenesis in individuals with Brca1 mutations. This study shows that DNA breaks in Brca1-deficient cells are aberrantly joined into complex chromosome rearrangements through a process dependent on nonhomologous end-joining (NHEJ) factors 53BP1 and DNA ligase 4 (Lig4). Loss of 53BP1 alleviates hypersensitivity of Brca1 mutant cells to PARP inhibition and restores error-free repair by HR. Mechanistically, 53BP1 deletion promotes ATM-dependent processing of broken DNA ends to produce recombinogenic single-stranded DNA competent for HR. In contrast, Lig4 deficiency does not rescue the HR defect in Brca1 mutant cells but prevents the joining of chromatid breaks into chromosome rearrangements. Our results illustrate that HR and NHEJ compete to process DNA breaks that arise during DNA replication, and shifting the balance between these pathways can be exploited to selectively protect or kill cells harboring Brca1 mutations. Brca1 mutations predispose carriers to breast and ovarian cancer. In the absence of Brca1, Xrcc2, or other HR proteins, Rad51 foci formation and homology-dependent repair are impaired. Defects in HR result in accumulation of chromatid breaks, leading to reliance on alternative repair pathways that can join DSBs on different chromatids to produce complex chromosomal rearrangements. Genomic instability following Brca1 loss is hypothesized to be a key factor in tumorigenesis, but additional mutations are required for survival and outgrowth of tumor cells. HR-deficient cells are sensitive to PARP inhibitors, which prevent repair of ssDNA breaks, converting them into DSBs during DNA replication. These breaks are normally repaired by Rad51-dependent HR using the sister chromatid as a template. PARP inhibitors are used in clinical trials for breast and ovarian cancers with Brca1 or Brca2 mutations. However, Brca2-deficient tumors can acquire reversion mutations that enable resistance to chemotherapeutic agents. Mice homozygous for the exon 11 deletion of Brca1 die in utero. Embryonic cell death is associated with apoptosis and activation of the ATM-Chk2-p53 pathway. Deletion of 53BP1 rescues the viability of Brca1 mutant mice, but Brca1/53BP1 double-deficient mice exhibit reduced tumorigenesis and near-normal lifespan. However, these cells show elevated DSBs and intact ATM-Chk2-p53 signaling. The only known functions of 53BP1 are in transducing ATM-dependent cell-cycle checkpoints and facilitating the joining of distal53BP1 Inhibits Homologous Recombination in Brca1-Deficient Cells by Blocking Resection of DNA Breaks Defective homologous recombination (HR) is a major contributor to tumorigenesis in individuals with Brca1 mutations. This study shows that DNA breaks in Brca1-deficient cells are aberrantly joined into complex chromosome rearrangements through a process dependent on nonhomologous end-joining (NHEJ) factors 53BP1 and DNA ligase 4 (Lig4). Loss of 53BP1 alleviates hypersensitivity of Brca1 mutant cells to PARP inhibition and restores error-free repair by HR. Mechanistically, 53BP1 deletion promotes ATM-dependent processing of broken DNA ends to produce recombinogenic single-stranded DNA competent for HR. In contrast, Lig4 deficiency does not rescue the HR defect in Brca1 mutant cells but prevents the joining of chromatid breaks into chromosome rearrangements. Our results illustrate that HR and NHEJ compete to process DNA breaks that arise during DNA replication, and shifting the balance between these pathways can be exploited to selectively protect or kill cells harboring Brca1 mutations. Brca1 mutations predispose carriers to breast and ovarian cancer. In the absence of Brca1, Xrcc2, or other HR proteins, Rad51 foci formation and homology-dependent repair are impaired. Defects in HR result in accumulation of chromatid breaks, leading to reliance on alternative repair pathways that can join DSBs on different chromatids to produce complex chromosomal rearrangements. Genomic instability following Brca1 loss is hypothesized to be a key factor in tumorigenesis, but additional mutations are required for survival and outgrowth of tumor cells. HR-deficient cells are sensitive to PARP inhibitors, which prevent repair of ssDNA breaks, converting them into DSBs during DNA replication. These breaks are normally repaired by Rad51-dependent HR using the sister chromatid as a template. PARP inhibitors are used in clinical trials for breast and ovarian cancers with Brca1 or Brca2 mutations. However, Brca2-deficient tumors can acquire reversion mutations that enable resistance to chemotherapeutic agents. Mice homozygous for the exon 11 deletion of Brca1 die in utero. Embryonic cell death is associated with apoptosis and activation of the ATM-Chk2-p53 pathway. Deletion of 53BP1 rescues the viability of Brca1 mutant mice, but Brca1/53BP1 double-deficient mice exhibit reduced tumorigenesis and near-normal lifespan. However, these cells show elevated DSBs and intact ATM-Chk2-p53 signaling. The only known functions of 53BP1 are in transducing ATM-dependent cell-cycle checkpoints and facilitating the joining of distal