ERBB2 and breast cancer: Although HER2-targeted therapies are not effective in patients with HER2-low-expressing tumors (Fehrenbacher et al. 2020), recent studies have shown promising therapeutic activity using HER2-directed antibody–drug conjugates (ADCs) with chemotherapeutics, such as trastuzumab deruxtecan (T-DXd) and trastuzumab duocarmazine (SYD985), in HER2-low breast cancer patients (Rinnerthaler et al. 2019; Modi et al. 2020).