SMAD is a family of proteins similar to the gene products of the Drosophila gene 'mothers against decapentaplegic' (Mad) and the C. elegans gene Sma. SMAD proteins are signal transducers and transcriptional modulators that mediate multiple signaling pathways. They mediate the signal of the transforming growth factor (TGF)-beta, and thus regulate multiple cellular processes, such as cell proliferation, apoptosis, and differentiation.