Intratumoral heterogeneity has been observed as early as studies developing initial HER2 IHC testing, with the heterogeneity of tumor cell HER2 IHC staining observed to be greater in gastroesophageal compared to breast adenocarcinomas, with staining also more often seen in a basolateral and less complete membranous pattern than breast cancer [37]. Here, ERBB2 is linked to breast cancer.