SMAD2 and plasma cell myeloma: SB431542, is an inhibitor of ALK4, ALK5 and ALK7, the TGF-β family type 1 receptors that preferentially activate SMAD2 and/or SMAD3.[20] We have previously shown that SB431542 inhibited activin A- and TGF-β-induced SMAD2 phosphorylation also in myeloma cells,[17] and here we found that GDF15-induced activation of SMAD2 in the INA-6 myeloma cell line was inhibited by SB431542 (Fig 1B).