Inhibitor of DNA binding 1 (Id1) is a basic helix-loop-helix (bHLH) protein that has a variety of functional roles in cellular events including differentiation, cell cycle and cancer development. In addition, it has been demonstrated that Idl is related with TGF-beta and Smad signaling in various biological conditions. In this study, we investigated the effect of Idl on TGF-beta-induced collagen expression in human dermal fibroblasts. When ldl-b isoform was overexpressed, TGF-beta-induced collagen expression was markedly inhibited. Consistent with this result, Idl-b significantly inhibited TGF-beta-induced collagen gel contraction. In addition, Id1-b inhibited TGF-beta-induced phosphorylation of Smad2 and Smad3. Finally, immunohistochemistry showed that Idl expression was decreased in fibrotic skin diseases while TGF-beta signaling was increased. Together, these results suggest that Idl is an inhibitory regulator on TGF-beta-induced collagen expression in dermal fibroblasts. (C) 2014 Elsevier Inc. All rights reserved.