In this pilot study, we used material from previously classified HER2-positive GC resection specimens to quantitatively and qualitatively assess the heterogeneity of the histological subtype and HER2 expression in different regions of the tumor to identify the minimum number of biopsy specimens needed to achieve the highest concordance between biopsy specimen and resection specimen HER2 status. Here, ERBB2 is linked to gastric cancer.