53BP1 loss promotes the end resection of DNA DSBs in the absence of BRCA1, resulting in RAD51 recruitment and HR restoration.31, 32, 93 Compared with the parental BRCA1‐deficient breast cancer cells, breast cancer cells with dual 53BP1 and BRCA1 deficiency displayed a reduced sensitivity of up to 36‐fold to PARP inhibitors. Here, PARP1 is linked to breast cancer.