– Activin RIA, also known as ALK-2, TSK-7L, SKR1, TSR-I, and ACTR-I, is a glycosylated 65 kDa type I receptor in the TGF-beta serine/threonine kinase receptor family. Binding of TGF-beta superfamily ligands induces formation of a heterotetrameric complex that contains two chains each of a type I and a type II receptor in multiple combinations. The type II receptors phosphorylate the type I receptors which then phosphorylate and activate Smad signal transduction proteins. Activin RIA functions in a wide variety of growth and differentiation processes including gastrulation, skeletal system development, and cardiac morphogenesis. BMP signaling through Activin RIA is enhanced by the direct interaction between Activin RIA and RGM-B/DRAGON, a BMP coreceptor that also associates with other type I and type II receptors. Activin RIA can additionally phosphorylate the coreceptor Endoglin and is required for the inhibitory effect of Endoglin on prostate cancer cell motility.