I'm guessing you meant myostatin. I researched this a while back after seeing a kid on tv with a myostatin disorder that allowed him to carrie 40% more muscle than the average person. This kid was 4 or so and had strength of kids twice his age and extremely low body fat to the point where he was always eating something with fat in it.
But in my research this is what I found:
Follistatin can cause localized growth in a muscle group. There was a study done where follistating was injected into the quad of an ape and the quad grew visibly bigger (don't remember the %)
Myostatin Inhibitor were made for people with muscular dystrophy disorders. It is administered through an IV and you are changing your genes. Once using a myostatin inhibitor the results are permanent and irreversible. With the excessive muscle growth from a myostatin inhibitor on a regular person it can cause brittle ligaments (forgot the exact reason why)
Also in early testing of myostatin inhibitors on apes, the inhibitor turned the apes blood into a thick gel (which I assume killed the ape)
I don't think I would touch one even if i had the opportunity because I want to have control over my body and I don't want to do something that is cannot be undone.

Also any myostatin inhibitor supplements that are advertised are scams because to inhibit your myostatin you have to change your genes. Save your money and get something that has been proven to work.