The stem cell-enriched basal population was expanded in MMTV-Wnt1 mammary tumors, but not when MMTV was used to drive the expression of Hras, Erbb2 (also known as Neu and Her2), or Polyoma middle T antigen (PyMT) [104,215], consistent with the idea that Wnt signaling activity is essential for stem cell maintenance. Here, ERBB2 is linked to breast cancer.