In our study, the HER2-overexpressing GC cell lines NCI-N87 and OE‐19 were insensitive to DIACC3010 monotherapy, possibly due to DIACC3010-induced pERK elevation, whereas DIACC3010 + trastuzumab synergistically inhibited in vitro growth of OE-19 cells, colony formation, and in vivo growth of OE-19 tumors in mice. This evidence concerns the gene ERBB2 and gastric cancer.