TGFB1 and neoplasm: When tumor cells are in close proximity to each other, mesenchymal fibroblasts become activated into cancer-associated fibroblasts (CAFs), which secrete cytokines such as TGF-β1 and platelet-derived growth factor.[21] CAF-generated cancer-promoting molecules may function as paracrine molecules, which attach to receptors on the surfaces of nearby cancer cells and accelerate tumor growth.