 Drug-based antagonists have also been identified, such as SB431542, which selectively inhibits ALK4, ALK5, and ALK7.The transforming growth factor receptor 3 (TGFBR3) is the most abundant of the TGF-β receptors yet, it has no known signaling domain.It however may serve to enhance the binding of TGF beta ligands to TGF beta type II receptors by binding TGFβ and presenting it to TGFBR2.