It is difficult to identify cancer driver genes in cancers, for instance BRCA1 mutated breast cancer, that are characterised by large scale genomic alterations. Here, the authors develop genetically engineered mouse models of BRCA1-deficient breast cancer that allow highthroughput in vivo perturbation of candidate driver genes, validating drivers Myc, Met, Pten and Rb1, and identifying MCL1 as a collaborating driver whose targeting can impact efficacy of PARP inhibition.