Although TGF-β is a major regulator of tumorigenesis and TGF-β signaling is present in nearly all cells, an ambiguous function of TGF-β as a tumor suppressor in early-stage tumors and a tumor enhancer in advanced cancer poses a challenge in developing therapeutics that target TGF-β for the treatment of breast and prostate cancer metastases. This evidence concerns the gene TGFB1 and prostate cancer.