Furthermore, in various mixed (HER2‐overexpressing, HER2‐low, or HER2−) cell cultures, the cytotoxic photo‐bystander effect was observed on nontargeted HER2− or HER2‐low cells (Figure S8), suggesting that this effect of T‐DM1–IR700‐mediated NIR‐PIT exerted across the cancer cell types. This evidence concerns the gene ERBB2 and cancer.