HER2-positive advanced gastric cancer has been found not only to share some of mechanisms of resistance with breast cancer, but also to manifest specific mechanisms of resistance to trastuzumab, including tumor heterogeneity in HER2 positivity, loss of HER2 protein expression, alteration in HER2 downstream signaling, and activation of bypass pathways [9]. This evidence concerns the gene ERBB2 and breast carcinoma.