The NRG-HER3-HER2 axis has been strongly implicated in HER2+ breast cancer relapse [10,17,27,30,32,61], and preclinical studies have shown that pertuzumab and LJM716 (HER3-inactivating mAb) significantly reduce the outgrowth of experimental HER2+BM [27]. This evidence concerns the gene ERBB3 and breast cancer.