SB431542 is a selective inhibitor of the TGF‐β1 receptor ALK5, whose molecule weight is 384.39.24 To address the association of overexpressing MMP‐9 with the activation of TGF‐β/SMAD signal pathway in various breast cancer cells, SB 431542 (10 μmol/L) was added into the cultured cells, alone or together with MMP‐9 or MMP‐9/TGF‐β. The gene discussed is TGFBR1; the disease is breast cancer.