ERBB2 and neoplasm: To confirm the in vivo cytotoxic photo‐bystander effect of NIR‐PIT with T‐DM1–IR700, we established a mixed tumor model comprising HER2+ 3T3/HER2 and HER2− MDAMB‐468‐luc‐GFP cells, and the antitumor cytotoxic photo‐bystander effect (nontargeted tumor cells) in this mixed tumor model was monitored by measuring the luciferase activity of MDAMB‐468‐luc‐GFP cells.