To investigate this, we used mammary carcinoma MDA MB-468 cells that do not express endogenous Smad4 because of a deletion of the Smad4 gene and are therefore unresponsive to TGF-β.21 Cells were transfected with the TGF-β-specific reporter CAGA12-luciferase,28 and dual luciferase reporter gene assay experiments were carried out (Fig. 3). Here, SMAD4 is linked to breast carcinoma.