A soluble fusion protein composed of the extracellular domain of the activin receptor type IIA (ActRIIA) linked to the Fc portion of human IgG1 with anabolic bone activity. Sotatercept selectively binds to activin, inhibiting its binding to ActRIIA and ActRIIA signaling, resulting in the stimulation of osteoblast activity and the inhibition of osteoclast activity and so normal bone formation and increased bone mineral density and strength. The Fc moiety of this fusion protein binds to the salvage receptor FcRN, preventing its lysosomal degradation and so extending its half-life in the circulatory system.