BRCA1 and breast cancer: To study the contribution of BRCA1/2-independent PARPi resistance mechanisms in BRCA1/2-deficient tumors, we used two genetically engineered mouse models (GEMMs) of BRCA1-associated breast cancer, K14cre;Brca1F/F;Trp53F/F (KB1P) and K14cre;Brca1F/F;Trp53F/F;Mdr1a/b−/− (KB1PM), as well as a GEMM of BRCA2-associated breast cancer, K14cre;Brca2F/F;Trp53F/F (KB2P)8,9,38,39,40 (Figure 1A).