ERBB2 and breast cancer: Indeed, despite the paucity of data available in HER2-low breast disease, being a rather new and quite unexplored field, there is a strong rationale that, also in HER2-low tumors, different intrinsic subtypes may explain different clinical behavior and response to treatments, as this has been already observed for other breast cancer subtypes (i.e., HER2-positive, HR-positive, HER2-negative and triple-negative breast cancers) [13,14,15].