The activin signaling pathway is one of two branches of Transforming Growth Factor-β family signaling in Drosophila. The binding of an activin family dimer to a heterodimeric serine/threonine kinase receptor complex (composed of type I and type II subunits), results in the phosphorylation and activation of the type I receptor by the type II subunit. In the activin branch, the downstream target of the type I receptor is 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.)