Myostatin is a negative regulator of skeletal muscle hypertrophy, meaning that it inhibits muscle growth. Thus, a myostatin mutation can cause massive muscle growth. According to the recent news story by the Associated Press, a 19-month-old boy "suffering" from myostatin mutation already posses superhuman strength and ripped abs.  He could do the iron cross when he was only five months old, according to his adoptive mother. This little superhuman eats like there's no tomorrow without gaining a microgram of extra lard. Fortunately, myostatin does not appear to regulate heart muscle size. Some sports supplements claim to block myostatin, but there is no evidence supporting these claims. A recent study examined 12 weeks of resistance training and "myostation blocker" supplementation (cystoseira canariensis) on blood levels of myostatin and muscle strength and body composition. The results indicated that cystoseira canariensis supplementation is ineffective at inhibiting blood myostatin and increasing muscle strength and mass or decreasing fat mass.

References:

Cohn RD et al. Myostatin does not regulate cardiac hypertrophy or fibrosis. Neuromuscul Disord, 2007 Apr;17(4):290-6.   

Willoughby DS. Effects of an alleged myostatin-binding supplement and heavy resistance training on serum myostatin, muscle strength and mass, and body composition. Int J Sport Nutr Exerc Metab, 2004 Aug;14(4):461-72.