A protein complex composed of a single type of SMAD family proteins. In the absence of Smad4, phosphorylation of R-SMADs results in their homotrimerization. However, these complexes do not appear to import into the nucleus and are assumed to be transcriptionally inactive. [ http://www.ncbi.nlm.nih.gov/pubmed/11779505 http://www.ncbi.nlm.nih.gov/pubmed/16322555 http://www.ncbi.nlm.nih.gov/pubmed/9670020 ]