Receptors for activins are membrane protein kinases belonging to the family of protein-serine-threonine kinases, thus also named activin receptor-like kinases (ALK's). Activin receptors also bind transforming growth factor beta. As those transmembrane receptors of the TGF-beta superfamily (transforming growth factor beta receptors), ALK's consist of two different but related protein kinases, Type I and Type II. Activins initiate cellular signal transduction by first binding to the type II receptors (type II activin receptors ) which then recruit and phosphorylate the type I receptors (type I activin receptors ) with subsequent activation of the type I kinase activity.