ERBB2 and breast cancer: The recently developed aIHCntibody-drug conjugate (ADC) drug, T-DXd, has been approved for the treatment of patients with metastatic HER2-positive BC (Nakada et al., 2019) and it effectively targets HER2-low tumour cells and delivers its potent cytotoxic payload to neighbouring tumour cells via a bystander effect (Ogitani et al., 2016a; Ogitani et al., 2016b).