Consistent with this idea, studies on mammary tumors in mice revealed that enhanced TGF-β signaling leads to a delayed development of primary tumors but an increased formation of lung metastases whereas a disruption of TGF-β signaling has the opposite effect leading to earlier appearance of primary tumors but a lower number of metastatic foci (Roberts and Wakefield, 2003). This evidence concerns the gene TGFB1 and breast cancer.