To illustrate the role of miR-203 as an antitumor agent in vivo, we chose the PyMT breast cancer model for its close similarity to human breast cancer, exemplified by the fact that in these mice a gradual loss of steroid hormone receptors (estrogen and progesterone) and β1-integrin is associated with over-expression of ERBB2 and cyclin D1 in late-stage metastatic cancer [46]. This evidence concerns the gene ERBB2 and breast cancer.