Leptin is a protein that affects the regulation of metabolism. Leptin controls fat deposition and marbling in cattle by increasing appetite and decreasing metabolism. A CC marker for Leptin decreases appetite and increases metabolism. Feedlot operators know that Leptin does not affect the first two phases of the growth curve. As long as animals are grouped according to frame and their Leptin genotype, we can increase the rate of finish and marbling without affecting yield. The TT genotype is homozygous. Used on any group of cows, the CC genotype will be eliminated. 

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Dr. Eric DeVuyst, OSU Ag Economist wrote in an Oklahoma Extension Service circular - The Economics of Gene Testing... Genotype (TT) fed cattle are profitable than lean genotype (CC) cattle. Differences in profit range from $14 to $60 per head. 

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The same mutation has been investigated in milk production and beef calf weaning weight. DeVuyst et al. 2008 also reported heavier calf weaning weights for TT and CT cows. The effect was significant in crossbred cows. TT cows wean heavier calves than CC cows. Cross-bred TT cows wean calves weighing 27 pounds more than calves weaned from cross bred CC cows. Their analysis found that TT and CT cows were more profitable than CC cows due to higher calf weaning weights and longer productive lives. Differences in profitability ranged from $15 to $39 per year depending on breed.