In line with the extraordinary plasticity of luminal progenitors (44), other features of sEVHYP-induced mammary gland transformation are reminiscent of luminal breast cancer, including lower expression of ER, PR, and HER2 (4), which has profound implications for patient survival (45), and dramatic acceleration of MMTV-PyMT breast tumorigenesis (35), a widely accepted model of luminal disease (46). Here, ERBB2 is linked to breast cancer.