N-terminal Mad Homology 1 (MH1) domainThe MH1 is a small DNA-binding domain present in SMAD (small mothers against decapentaplegic) family of proteins, which are signal transducers and transcriptional modulators that mediate multiple signaling pathways. MH1 binds to the DNA major groove in an unusual manner via a beta hairpin structure. It negatively regulates the functions of the MH2 domain, the C-terminal domain of SMAD. Receptor-regulated SMAD proteins (R-SMADs, including SMAD1, SMAD2, SMAD3, SMAD5, and SMAD9) are activated by phosphorylation by transforming growth factor (TGF)-beta type I receptors. The active R-SMAD associates with a common mediator SMAD (Co-SMAD or SMAD4) and other cofactors, which together translocate to the nucleus to regulate gene expression. The inhibitory or antagonistic SMADs (I-SMADs, including SMAD6 and SMAD7) negatively regulate TGF-beta signaling by competing with R-SMADs for type I receptor or Co-SMADs. MH1 domains of R-SMAD and SMAD4 contain a nuclear localization signal as well as DNA-binding activity. The activated R-SMAD/SMAD4 complex then binds with very low affinity to a DNA sequence CAGAC called SMAD-binding element (SBE) via the MH1 domain.