Wyatt pharmaceuticals has had a monoclonal antibodies to myostatin in clinical trials since April 2005.
It is designed to be used in the treatment of various muscular dystrophies.
Called something like MYO-89.
There are also cows with this mutation, Belgian blue and Piedsmontses.
However before you get all excited, it is still uncertain how major a role myostatin plays in the hypertrophy of muscle. It is only the 'brakes' for muscle grow, which call also be naturally induced by higher levels of arginine/citrulline, which make nitric oxide, which induces follistatin, which in turn suppresses myostatin.
I'm surprised they haven't investigated this venue as a treatment for HIV. If they do, look out. Fuck Gh, IGF, slin, and most steroids. That shit is going to be POP-YOO-LAR!