Cell Line Characterization and Validation

All cell lines were passaged in αMEM supplemented with 5% fetal bovine serum (FBS), and insulin/transferrin/selenium supplement (Roche, Basel, Switzerland) without antibiotics for less than 3 months from frozen stocks confirmed to be mycoplasma-free. The human breast cancer estrogen receptor-positive MCF-7 and triple-negative MDA-MB-231 cell lines, and the human embryonic kidney cell line HEK293 were purchased from the American Type Culture Collection (ATCC). The Chinese hamster ovary CHO cell lines 51D1 and 51D1.3 were obtained from Prof. William Wilson. Their DNA repair genotypes and origins, were as follows: 51D1 [36 (link)] (rad51d knockout) and 51D1.3 [36 (link)] (51D1 complemented with CHO Rad51d). All cell lines tested negative for mycoplasma contamination (PlasmoTestTM—Mycoplasma Detection kit, InvivoGen, San Diego, CA, USA).