This is particularly important since recent antibody-drug conjugate (ADC) developments such as trastuzumab-deruxtecan (T-DXd) are also effective in low HER2 expressing advanced breast cancer, and may work independently of the magnitude and possible intra-tumoral heterogeneity of HER2 expression [[10], [11], [12], [13]]. Here, ERBB2 is linked to breast cancer.