The Belgian Blue has a natural mutation in the myostatin gene which codes for the protein, myostatin ("myo" meaning muscle and "statin" meaning stop).[5] Myostatin is a protein that inhibits muscle development. This mutation also interferes with fat deposition, resulting in very lean meat.[5] The truncated myostatin gene is unable to function in its normal capacity, resulting in accelerated lean muscle growth. Muscle growth is due primarily to physiological changes in the animal's muscle cells (fibers) from hypertrophy to a hyperplasia mode of growth. This particular type of growth is seen early in the fetus of a pregnant dam, which results in a calf that is born with two times the number of muscle fibers at birth than a calf with no myostatin gene mutation.[5] In addition, a newborn double-muscled calf’s birth weight is significantly greater than that of a normal calf.