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Roughly 2% to 3% of all infants are allergic to cow's milk. Although most babies eventually outgrow this allergy, it can pose a major problem for those who aren't breast-fed, since milk is used as a base in most formulas.

A solution to this problem may just reside in a single, tailless cow in New Zealand. This special calf, conceived through genetic modification and cloning, produces milk that contains no detectable levels of beta-lactoglobulin (BLG), the protein that is believed to trigger allergic reactions.

What's more, the hypoallergenic milk from this calf appears to be even more nutritious than regular cow's milk, as it contains double the amount of the healthy milk proteins known as caseins.

This experiment, detailed in a paper published today in the Proceedings of the National Academy of Sciences, marks the first time that scientists have successfully altered the protein composition of milk before it leaves the cow, says Mike Van Amburgh, Ph.D., an associate professor of animal science at Cornell University, in Ithaca, New York.

If bred in sufficient numbers, this type of genetically modified cow could one day provide milk for allergic infants and adults, according to the researchers in New Zealand who bred the calf. The team was led by Anower Jabed, Ph.D., a pharmacologist at the University of Auckland and a fellow at AgResearch, a government-funded institute for agriculture and food research.

Like other cloning efforts, the complex multi-year process that created the calf was an "incredibly difficult feat," says R. Michael Roberts, Ph.D., a professor of animal science and biochemistry at the University of Missouri-Columbia, who edited the paper.

The experiment began in mice, which are much easier than cows to work with in a laboratory: They're small, their gestational period is just 20 days (versus 290 for cows), and they produce litters of pups rather than just one or two offspring.

Jabed and his colleagues genetically altered mice to make them produce BLG, which is normally found only in cattle and sheep. A second alteration, which down-regulated (or suppressed) certain genes, inhibited the production of BLG and produced the desired effect: female mice whose milk was almost entirely BLG-free.

The researchers then repeated the process in cows. First, they injected cow cells containing the same anti-BLG genetic tweak into dozens of unfertilized cow eggs. Then, after fertilization, they implanted the embryos into the uteruses of mature cows and waited for the calves to be born.

"It was classic cloning, just like the way they did for Dolly," says Roberts, referring to Dolly the sheep, the first cloned mammal, who was born in Scotland in 1996.

The first attempt was a failure, but the second batch of embryos led to four successful pregnancies, one of which produced a healthy female calf. She was, however, born without a tail -- a rare birth defect that the researchers suspect is related to the genetic engineering but not to the BLG modifications per se.

Using hormones, Jabed's team artificially induced lactation in the calf and discovered that her milk contained extra casein and virtually no BLG. That combination would probably make the milk a better substitute for breast milk in allergic babies, Roberts says.

Although this calf may hold the key to circumventing milk allergies, much more work remains to be done before hypoallergenic milk from genetically modified cows appears on supermarket shelves.

For starters, the researchers will need to breed the calf, study her progeny for the relevant genetic traits, and confirm that this second generation of cows produces BLG-free milk. And even if all that goes smoothly, the milk would have to pass several regulatory hurdles -- and the test of public opinion -- to reach the market.

"There would be a long way to go from having a cow on the ground to producing this desirable milk, and getting it approved, and having it accepted by the population as a substitute for regular cow's milk," Roberts says.

Few precedents exist for this type of genetically engineered food product, Roberts adds. Some drugs in use today are made from so-called transgenic animals, but so far no genetically modified animals or animal products have been sold for food. The nearest example is a patent-protected salmon containing genes from two other fish species that enable it to grow faster, but that fish has yet to be approved by the Food and Drug Administration.

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