HER3 signaling plays a crucial role in the development of human cancers that exhibit a drug resistance phenotype, including HER2-over-expressing breast cancer [11, 12], castration-resistant prostate cancer [57], platinum-resistant/refractory ovarian cancer [58, 59], and EGFR tyrosine kinase inhibitor (TKI)-resistant non-small cell lung cancer (NSCLC) [60, 61]. This evidence concerns the gene EGFR and breast cancer.