Activins form a subfamily of the Transforming Growth Factor(TGF)-β family - a group of extracellular ligands for the TGF-β-Receptor family. The binding of an activin family dimer to a heterodimeric serine/threonine kinase receptor complex results in the phosphorylation of Smox, a member of the Smad family. Smox forms a complex with the co-Smad, Med. This complex translocates into the nucleus and regulates the transcription of target genes in concert with other nuclear cofactors. (Adapted from FBrf0236482.)