HER2 overexpression is diagnosed with immunohistochemistry (IHC) and gene amplification in situ hybridization (ISH) according to the following guidelines: IHC 3+ or 2+/ISH+ (chromosome enumeration probe [CEP] 17 ratio ≥ 2) is defined as HER2-positive gastric cancer [3]. Here, ERBB2 is linked to gastric cancer.