myostatin is a growth factor protein that limits muscle tissue growth.
belgian blue bulls are famous for this.
"The Belgian Blue has a natural mutation of the gene that codes for myostatin, a protein that counteracts muscle growth. The truncated myostatin is unable to function in this capacity, resulting in accelerated lean muscle growth, due primarily to hyperplasia rather than hypertrophy. The defect in the breed's myostatin gene is maintained through linebreeding. This mutation also interferes with fat deposition, resulting in very lean muscle."