A soluble fusion protein containing the extracellular domain of the activin receptor type 2B (ACVR2B or ActRIIB) fused to a human Fc domain, with potential antineoplastic activity. Upon intravenous administration, STM 434 selectively binds to the growth factor activin A, thereby preventing its binding to and the activation of endogenous ActRIIB. This prevents activin A/ActRIIB-mediated signaling and inhibits the proliferation of activin A-overexpressing tumor cells. Activin A, a member of the transforming growth factor beta (TGF-beta) superfamily, is overexpressed in a variety of cancers and plays a key role in promoting cancer cell proliferation, migration, and survival. Check for active clinical trialsusing this agent.