Overexpression of Smad7 inhibits R-Smad phosphorylation and abrogates TGF-β–mediated growth inhibition. (A) Western blot analysis performed on BM cells from recipient mice. Protein was extracted from BM cells stimulated with the indicated ligands and subsequently used for Western blot analysis. Actin was used as control for loading. P-Smad indicates phosphorylated. (B) GFP⁺ cells were sorted after transduction and cultured for 6 days under serum-free conditions with or without increasing concentrations of active TGF-β1 as indicated. The growth was compared with cultures without exogenous TGF-β1 and calculated as percentage of values. Data represent mean values ± SEM from 3 independent experiments on day 6 of culture.