SMAD1 A gene on chromosome 4q31.21 that encodes a SMAD family protein, named for their similarity to the Drosophila gene Mothers Against Decapentaplegic (Mad), which are signal transducers and transcription modulators of multiple signalling pathways. SMAD1 mediates signals from bone morphogenetic proteins (BMPs), which mediate an array of biological activities, including cell growth, apoptosis, morphogenesis, development and immune responses. In response to the binding of BMP ligands, SMAD is phosphorylated and activated by the BMP receptor kinase; phosphorylated SMAD1 forms a complex with SMAD4, resulting in transcription regulation. SMAD1 is a target for SMAD-specific E3 ubiquitin ligases—SMURF1 and SMURF2. Its activity is downregulated by ubiquitination and proteasome-mediated degradation.