The importance of MAPK in mediating the oncogenic function of TGF-β is exemplified by the fact that the Smad4 phosphatase Wip1 can restrain the TGF-β-induced cell growth arrest, migration, and invasion and enhances the tumorigenicity of cancer cells by repressing Smad4 activity via antagonising MAPK function. Here, SMAD4 is linked to cancer.