This is somewhat unexpected because BC subtypes have beenlinked to the cell-of-origin within the mammary epithelial cell hierarchy; claudin-low tumors arethought to arise from transformation of mammary stem cells, basal-like tumors from bi-potent/earlyluminal progenitors, HER2+ BC from more committed luminal progenitors, etc. (Limet al, 2009; Prat & Perou,2009). Here, ERBB2 is linked to breast cancer.