Transforming growth factor-β (TGF-β) isoforms are growth factors that function physiologically to regulate development, cellular proliferation, and immune responses. TGF-β has been reported to function as both a tumor suppressor and tumor promoter. To elucidate the role of TGF-β signaling in mammary gland development, tumorigenesis, and metastasis, the gene encoding the type II TGF-β receptor, Tgfbr2, was conditionally deleted in the mammary epithelium (Tgfbr2^MGKO). Tgfbr2^MGKO mice were mated to the MMTV-polyomavirus middle T antigen (PyVmT) transgenic mouse model of metastatic breast cancer. Loss of Tgfbr2 in the context of PyVmT expression results in a shortened median tumor latency and an increased formation of pulmonary metastases. Pictured on the cover is a lung section illustrating the increased number of lung metastasis from a PyVMT/Tgfbr2^MGKO mouse. These studies support a tumor suppressive role for epithelial TGF-β signaling in mammary gland tumorigenesis and demonstrate that pulmonary metastases can occur and are even enhanced in the absence of TGF-β signaling in carcinoma cells. For details, see the article by Forrester et al. on page 2296 of this issue.