Source:http://linkedlifedata.com/resource/umls/id/C0246504
NCI: Activins are dimeric growth and differentiation factors that belong to the TGF-beta superfamily of signaling proteins. Activins signal through a heteromeric complex of receptor serine kinases which include at least two type I (I and IB) and two type II (II and IIB) receptors. Type I receptors are essential for signaling and type II receptors are required for binding ligands and for expression of type I receptors. Type I and II receptors form a stable complex after ligand binding, resulting in phosphorylation of type I receptors by type II receptors. Activin A type IIB receptor, encoded by the ACVR2 gene and a receptor for activin A, activin B, and activin C, displays a 3- to 4-fold higher affinity for the ligand than activin A type II receptor. (From LocusLink, Swiss_Prot)