Once T-DXd binds to the HER2 protein, irrespective of the level of HER2 expression, it transports its toxic cargo (8:1 DAR) to cancer cells and adjacent cells through a bystander effect (Ogitani et al., 2016a; Ogitani et al., 2016b), which distinguishes T-DXd from conventional HER2-targeted therapies (Figure 2). Here, ERBB2 is linked to cancer.