Growth and differentiation factor-associated serum protein-1 (GASP-1) binds to mature myostatin, a member of the TGFb superfamily, and myostatin propeptide. GASP-1 possesses multiple domains associated with protease-inhibitor proteins, including a whey acidic protein domain, a Kazal domain, two Kunitz domains, and a netrin domain. It also contains a highly conserved module of cysteine-rich sequence termed the follistatin domain. Ubiquitous GASP-1 overexpression in mice leads mainly to a hypermuscular phenotype. It has been described that latent myostatin has significant activity and the binding to the myostatin receptor is controlled more efficiently by WFIKKN1 (GASP-2) than by WFIKKN2 (GASP-1). In that sense, GASP-2 may have greater potential as an antimyostatic agent than GASP-1. The identification of myostatin-binding proteins capable of regulating myostatin activity expands the number of potential therapeutic targets.