ERBB2 and neoplasm: An important challenge with HER2-targeting drugs for GC is the heterogeneity of HER2 (e.g., protein expression variations and gene copy number alterations) within cancers (Zhu et al. 2021). Many GC cases may exhibit varying levels of HER2 positivity, and even within a single tumor, there can be areas that are HER2-positive and others that are HER2-negative (Park et al. 2016; Cho et al. 2013).