has role Smad R-Smad There are five vertebrate R-Smads, Smad1, Smad2, Smad3, Smad5 and Smad8 and they are phosphorylated in response to different types of TGF-beta ligands. Phosphorylated R-Smads can form complexes with Smad4 and positively or negatively regulate transcription. The Mad protein in Drosophila is the ortholog of Smad1/5, whereas dSmad2 is the ortholog of Smad2/3. In C. elegans, sma-2 and sma-3 proved to be homologous to Mad of Drosophila. receptor-activated Smad receptor-regulated Smad transcription regulator