on them. The myostatin gene is found in all mammals and influences the production of a protein that controls muscle development. Variants of the gene produce proteins that are less effective at controlling muscle development, which results in increased muscle mass. Limousin muscling is intermediate to that of British cattle breeds such as Aberdeen Angus, Hereford, and Shorthorn and the extreme double muscling found in the European Belgian Blue and Piedmontese breeds. Studies of double-muscled cattle identified natural mutations of the myostatin gene which produce inactivated proteins that are unable to control muscle development. In Belgian Blue and Piedmontese cattle this causes an increase in muscle mass of 20–25%. Subsequent studies identified a less extreme myostatin mutation known as F94L associated with Limousins. The resulting partially active protein results in Limousins having intermediate muscle development, which avoids the extreme muscling and associated disadvantages of double muscled cattle. A Limousin/Jersey backcross study conducted in Australia and New Zealand to investigate the effects of the F94L myostatin variant concluded that the mutation had no significant effect on birth-weight and growth traits. Averaged over all backcross calves in the trial (total of 766), animals homozygous for the mutation had approximately 6% heavier carcases than animals without the mutation, 15% larger eye muscle (also known as rib eye) area, 13% heavier silverside weight, and 13% heavier total meat weight. Increased meat weight and size was accompanied by a 15% reduction in intra-muscular fat and 25% reduction in total fat weight. No other significant effects were observed. A second backcross study conducted in Japan of Limousin and Japanese Black breeds identified similar changes to meat and fat quantities in cattle homozygous for the F94L mutation. Although the Australian/New Zealand study found that the F94L mutation was partially to significantly recessive in most traits, meaning cattle heterozygous for the mutation express less to significantly less than half of the effects noted for homozygous cattle, the Japanese study found that the meat and fat quantities in cattle heterozygous for the mutation were about midway between the two extremes. Distribution of F94L myostatin variants in Limousin cattle A number of international breed associations have been testing the F94L status of cattle registered in their herd books. The absence of F94L genes in some tested cattle might be a result of a sire or dam ancestor that had double muscling (MH) genes, or more likely that the myostatin gene