Purpose To review the recent developments in the structure and function of Smad proteins Data Sources Both Chinese- and English-language literatures were searched using MEDLINE/CD -ROM (1997-2000) and the Index of Chinese-Language Literature (1997-2000).Study Selection Data from published articles about TGF-β signal transduction in recent domesti c and foreign literature were selected. Data Extraction Data were mainly extracted from 22 articles which are listed in the reference se ction of this review. Results Smad proteins mediate signal transduction induced by the TGF-β superfamily. B ased on their structural and functional properties, Smad proteins are divided in to three groups. The first group, receptor-regulated Smads (R-Smads), are pho sphorylated by activated type Ⅰ receptors and form heteromeric complexes with t he second group of Smads, common mediator Smads (Co-Smads). These Smad complex es translocate into the nucleus to influence gene transcription. Inhibitory Sma ds (I-Smads) are the third group and these antagonize the activity of R-Smads . In the nucleus, Smads can directly contact Smad-binding elements (SBE) in ta rget gene promoters. Through interaction with different transcription factors, transcriptional co-activators or co-repressors, Smads elicit different effects in various cell types. The aberrance of Smad proteins has been noted in severa l human disorders such as fibrosis, hypertrophic scarring and cancer. Conclusion The structure of Smads determines their function as transcriptional factors whic h translocate signals from the cell surface to the nucleus where Smads regulate TGF-β superfamily-dependent gene expression. Purpose To review the recent developments in the structure and function of Smad proteins Data Sources Both Chinese- and English-language literatures were searched using MEDLINE / CD-ROM (1997-2000) and the Index of Chinese-Language Literature (1997-2000) . Study Selection Data from published articles about TGF-β signal transduction in recent domesti c and foreign literature were selected. Data Extraction Data were mainly extracted from 22 articles which are listed in the reference setion of this review. Results Smad proteins mediate signal transduction induced by the TGF-β superfamily. B ased on their structural and functional properties, Smad proteins are divided into three groups. The first group, receptor-regulated Smads (R-Smads), are pho sphorylated by activated type I receptors and form heteromeric complexes with he second group of Smads, common mediator Smads (Co-Smads). These Smad complex es translocate into the nucleus to influence gene transcription. Inhibitory Sma ds (I-Smads) are the third group and these antagonize the activity of R-Smads. In the nucleus, Smads can directly contact Smad-binding elements (SBE) in ta rget gene promoters. Through interaction with different transcription factors, transcriptional co -activators or co-repressors, Smads elicit different effects in various cell types. The aberrance of Smad proteins has been noted in severa l human disorders such as fibrosis, hypertrophic scarring and cancer. Conclusion The structure of Smads determines their function as transcriptional factors whic h translocate signals from the cell surface to the nucleus where Smads regulate TGF-β superfamily-dependent gene expression.