Soluble form of the activin type IIB receptor (ActRIIB) fused to an IgG1 Fc domain. As a decoy receptor it binds and neutralizes myostatin along with other TGF-beta superfamily members that ordinarily limit muscle growth. Acceleron Pharma developed it for Duchenne muscular dystrophy (DMD) and took it to Phase 2 clinical trials, where vascular side effects — nosebleeds and telangiectasia among them — halted development. A single dose in healthy volunteers produced significant gains in lean mass and drops in fat mass within 29 days.