FDA-approved drugs for breast cancer in 2019 include the antibody–drug conjugate Fam-trastuzumab deruxtecan [37] (HER2-directed antibody and topoisomerase inhibitor) for the treatment of unresectable or metastatic HER2-positive breast cancer [38], the phosphoinositide-3-kinase (PI3Kα) inhibitor alpelisib [39] for the treatment of HER2-negative, PIK3CA-mutated, advanced or metastatic breast cancer [40] and in 2020, tucatinib, an orally bioavailable, small molecule tyrosine kinase inhibitor for patients with HER2-positive metastatic breast cancer [41]. Here, ERBB2 is linked to breast cancer.