To examine the effects of altered miR‐223 expression on the antitumor activity of trastuzumab in GC cells, cytotoxicity assays were carried out in NCI‐N87 parent cells transfected with mimic‐miR‐223 or mimic‐NC. Although trastuzumab exhibited antitumor activities in the NCI‐N87 parent cells, overexpression of miR‐223 reduced the sensitivity of the cells to trastuzumab significantly (p  < 0.001, Fig. 3d ).