The signaling pathways activated by PyV-mT include those of Ras, Shc, and phosphotidylinositol 3-kinase, which are frequently activated in human breast cancer (reviewed in [6]) and are also activated by ErbB2 (Her2/neu), a receptor tyrosine kinase that is overexpressed in 30% of breast cancer and is associated with poor outcome [7]. Here, ERBB2 is linked to breast cancer.