We found that the level of IL-6 was extremely high after trastuzumab treatment for six weeks when the STAT3 signaling became predominant survival signaling pathway in the resistant gastric cancer cells, implicating that the release of IL-6 that drives the STAT3 activation in response to trastuzumab initiates the survival signaling transition, which contributes to resistance to trastuzumab via a feedback mechanism. The gene discussed is IL6; the disease is gastric cancer.