HER2 testing in gastric cancer reveals important differences compared to breast cancer: i) in breast, full membrane staining is mandatory for 2+/3+ evaluation, while in gastric cancer lateral or basal staining is sufficient; ii) differently from breast cancer, gastric cancer often stains heterogeneously; thereof, a threshold of 10% positive cells was considered appropriate to assess HER2 status (30% for breast tumors). This evidence concerns the gene ERBB2 and breast cancer.