To validate the role of MDC1 in PARPi resistance in BRCA2-defective cells, we used an independent Brca2-/-;Trp53-/- mouse mammary tumor cell line (KB2P1.21) to generate polyclonal KB2P1.21 cell lines by targeting the Mdc1 gene with two different gRNAs. Here, TP53 is linked to breast cancer.