ERBB2 and neoplasm: Moreover, quantification of tumor volumes to compare the antitumor effects on whole mixed tumors indicated that T‐DM1–IR700‐mediated NIR‐PIT exerted a strong antitumor effect on whole mixed tumors, suggesting its antitumor effects on both the target 3T3/HER2 cells and nontarget MDAMB‐468‐luc‐GFP cells and confirming the in vivo cytotoxic photo‐bystander effect (**p = 0.0005, Kruskal–Wallis test with Dunn's post hoc test, n = 8 mice per group) (Figure 4g).