In line with this model, MDC1 loss also causes PARPi resistance in BRCA1-deficient tumor cells without restoring RAD51 foci formation, a typical hallmark of HR restoration that we frequently found in PARPi-resistant KB1P tumors harboring mutations in the 53BP1-RIF1-REV7-CST-Shieldin pathway [15–18]. This evidence concerns the gene MDC1 and neoplasm.