TGF-β plays an important role in regulating cell differentiation and proliferation in human cancers such as colorectal cancer. Id-1 has been identified as a marker in colorectal cancer progression. The aim of this study was to investigate the role of TGF-β in regulating Id-1 in LoVo cells. siRNA was used to silence smad2, smad3, and p38 MAPK gene expression in Lovo cells. Interference efficiency and the role of TGF-β on Id-1 expression were analyzed using a luciferase reporter assay, RT-PCR, and Western blotting. Cell viability was determined using the MTT assay. In this study, we demonstrated that TGF-β1 downregulated Id-1 protein expression in LoVo cells. Smad2 and smad3 siRNA inhibited TGF-β1-induced 4×SBE luciferase reporter activity. p38 MAPK siRNA inhibited TGF-β1-induced 3×AP-1 luciferase reporter activity. However, the suppression of Id-1 by TGF-β1 was recovered by smad3 siRNA but not smad2 or p38 MAPK siRNA. Moreover, TGF-β1 stimulated cellular proliferation and p21Waf1 protein expression, which might be mediated by suppressing Id-1 expression. In conclusion, this study demonstrated that TGF-β1 suppressed Id-1 expression in a smad3-dependent manner in LoVo cells using RNAi technology. These results provide new insight into the mechanisms of TGF-β function in colorectal cancer cells. Anat Rec, 2010. © 2009 Wiley-Liss, Inc.