ERBB2 and breast carcinoma: Intratumor heterogeneity involving breast cancer subtypes has been indirectly implicated to explain differences in outcomes when PAM50 and IHC subtype classifications are discordant in a primary tumor [19], when subtypes are discordant between a synchronous primary tumor and metastases [20], or when comparing treatment response for HER2 cases with or without ER positivity [21].