Follistatin is fascinating protein that can increase muscle mass beyond natural potential by suppressing myostatin. Scientists first identified follistatin while examining porcine (re: pig) follicular fluid in the ovaries. 
Follistatin is naturally found in the skeletal muscle of almost all mammals with advanced or developed characteristics, such as humans, rodents, and cows. Follistatin is high in the non-essential amino acid cystine but unlike most proteins discussed in the fitness world, follistatin has carbohydrates attached to it. 
Follistatin, specifically Follistatin 344 (FS344), quickly gained popularity in the bodybuilding community as a potential supplement to rapidly increase lean tissue mass. Another protein, follistatin-related gene (FLRG) acts on similar pathways as FS344 regarding its muscle building properties. Increased lean tissue mass could give a bodybuilder an advantage in a competitive setting and be the differentiating factor between first and second place.