ACE-031 is a soluble form of the activin type IIB receptor (ActRIIB) fused to an IgG1 Fc domain. It functions as a decoy receptor, binding and neutralizing myostatin and other TGF-beta superfamily members that normally limit muscle growth. Originally developed by Acceleron Pharma for Duchenne muscular dystrophy (DMD), ACE-031 reached Phase 2 clinical trials before development was halted due to vascular side effects including nosebleeds and telangiectasia. In healthy volunteers, a single dose produced significant increases in lean mass and reductions in fat mass within 29 days.