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 (receptors, transforming growth factor beta), 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 (activin receptors, type ii ) which then recruit and phosphorylate the type I receptors (ACTIVIN RECEPTORS, TYPE I ) with subsequent activation of the type I kinase activity.