Smad7 is an antagonist of TGF-beta signaling pathway and the mechanism of its inhibitory effect is of great interest. We recently found that Smad7 could function in the nucleus by binding to the DNA elements containing the minimal Smad binding element CAGA box. In this work, we further applied single-molecule force spectroscopy to study the DNA-binding property of Smad7. Smad7 showed similar binding strength to the oligonucleotides corresponding to the CAGA-containing activin responsive element (ARE) and the PAI-1 promoter, as that of Smad4. However, Smad7 also exhibited a binding activity to the mutant ARE with the CAGA sequence substituted, indicating its DNA-binding specificity is different from other Smads. Moreover, we demonstrated that the MH2 domain of Smad7 had a higher binding affinity to the DNA elements than the full-length Smad7, while the N-terminal domain exhibited an inhibitory effect. (C) 2008 Elsevier Inc. All rights reserved.