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.