ERBB2 and breast cancer: There are three widely-recognized breast cancer molecular subtypes, “luminal” (ER+/HER2−) [1], [2], [3], [4], “HER2-enriched” (HER2+) [5], [6] and “basal-like” (ER−/HER2−) [6], [7], [8], [9] and a considerable body of work has focused on defining prognostic signatures in these [10], [11].