This is because the original tumor directly resulted from RRAS2 overexpression; an advantage not available in established BC cell lines, even if they overexpress RRAS2. We demonstrate here that reducing RRAS2 expression four-fold in the CBM-MBC21 cell line slows proliferation in vitro due to a delayed progression through the S-phase of the cell cycle and diminishes the cells’ ability to form tumors in orthotopic locations. This evidence concerns the gene RRAS2 and breast cancer.