ERBB2 and gastric cancer: HER2 overexpression is found in approximately 7–34% of gastric cancers, depending on location and histological subtype, and is a predictive marker for response to trastuzumab, which is currently the only approved HER2-targeted therapeutic option for the first line treatment of patients with metastatic gastric cancer (Bang et al. 2010; Brien et al. 1998; Chung et al. 2009; Gravalos and Jimeno 2008; Park et al. 2006; Rüschoff et al. 2012; Takehana et al. 2002; Tanner et al. 2005).