The basic building blocks of the TGF-beta pathway are ubiquitous in animals, but given that it is a regulatory pathway, the functions of the pathways vary depending on the organism and cell type. C. elegans has two TGF-beta superfamily signaling pathways that incorporate ligands, receptors, and Smads. There is an interesting ligand,
unc-129, that does not appear to require conventional TGF-beta superfamily receptors and Smads. In addition, two other TGF-beta superfamily ligands have been identified. In some cases, aspects of TGF-beta signaling have been modified in C. elegans.