TGFB1 and cancer: In cancers, activated TGF-β ligands bind to TGF-β type II receptors and phosphorylate TGF-β type I receptors, which are an essential component of this bipartite transmembrane receptor; then phosphorylated TbRI subsequently phosphorylates intracellular Smad2 and Smad3 (Smad2/3).