ERBB2 and neoplasm: In model 1, it was observed that most invasive breast tumors can be arranged along the differentiation hierarchy of normal breast ductal epithelium, starting from the claudin-low subtype closest to Mammary Stem Cells (MaSCs), followed by basal-like and HER2-enriched subtypes, and then the two luminal tumor subtypes, with the luminal subtypes closest to mature luminal cells [129].