was the normal or wild type variant found most commonly in beef cattle. In the latter case, loss of the F94L variant will arise when grading up to purebred when base animals are not Limousins. Europe Of the 14 Limousins tested during the research that led to the discovery of the F94L variant, 12 cattle were homozygous for the variant and two were heterozygous. The second myostatin genes in both heterozygous cases were each different myostatin MH variants of types normally found in Belgian Blue and Charolais cattle. Britain British test results of sale bulls in February 2010 indicated that of 142 animals tested, just under 90% were homozygous for the F94L mutation, about 8.5% were heterozygous, and 1.5% did not have the mutation. USA Test results of approximately 1,100 cattle recorded in the North American Limousin Foundation herd book show the following distributions for three classes of animal. About 94.4% of Fullbloods, 62.3% of purebreds and 5.3% of Lim-Flex were homozygous for the F94L mutation. Australia and New Zealand Test results of 1028 cattle recorded in the Australian and New Zealand herd book indicate that 96.7% of Fullbloods (known in Australia as French Pure), 88.0% of purebreds, and 33.3% of a limited sample of Lim-Flex were homozygous for the F94L mutation. Implications for cattle heterozygous for the F94L variant Cattle heterozygous for the F94L myostatin mutation have a 50% probability of passing the mutation to their progeny. Because the mutation has greatest effect on carcase traits, only 50% of progeny of a heterozygous parent will inherit increased muscling associated with the mutation. Furthermore, best linear unbiased prediction (BLUP) techniques used to estimate the genetic merit of stud cattle (for example, estimated breeding values (EBVs) and expected progeny differences (EPDs)) will be incorrect because they assume that no dominant genes contribute to modelled traits. Inconsistent inheritance of myostatin mutations (for example, F94L in Limousins, nt821 in Angus, and Q204X in Charolais) by progeny is expected to result in possible BLUP prediction errors for EBVs and EPDs equalling or exceeding worst case standard errors of prediction. For example, average rib eye area for Limousins in US Meat Animal Research Center (USMARC) trials during the 1980s and early 1990s is reported to be 12.3in2, and the reported possible difference in rib eye area in progeny arising from inheritance of either two F94L mutations or two normal myostatin genes from heterozygous parents is