ACE-031 is an investigational protein therapeutic that builds muscle and increases strength by inhibiting signaling through a cell surface receptor called activin receptor type IIB (ActRIIB). ACE-031 is a recombinant fusion protein that is produced by joining a portion of the human ActRIIB receptor to a portion of a human antibody. This creates a freely circulating, decoy version of ActRIIB which removes proteins, such as GDF-8 (myostatin), that limit the growth and regeneration of muscle. Animal studies with ACE-031 suggest that blocking signaling through ActRIIB leads to increased muscle mass and improved physical function in a range of animal models of muscle disease, including models of muscular dystrophy. ACE-031 is currently being studied in a phase 2 clinical trial of patients with Duchenne Muscular Dystrophy (DMD) who are also receiving corticosteroid therapy.