This little piggy's manure causes less pollution. This little piggy produces extra milk for her babies. And this little piggy makes fatty acids normally found in fish, so that eating its bacon might actually be good for you.
The three pigs, all now living in experimental farmyards, are among the genetically engineered animals whose meat might one day turn up on American dinner plates. Bioengineers have also developed salmon that grow to market weight in about half the typical time, disease-resistant cows and catfish needing fewer antibiotics, and goats whose milk might help ward off infections in children who drink it.
Only now, though, do federal officials seem to be getting serious about drafting rules that would determine whether and how such meat, milk and filets can safely enter the nation's food supply.
Some scientists and biotechnology executives say that by having the Food and Drug Administration spell out the rules of the game, big investors would finally be willing to put up money to create a market in so-called transgenic livestock.
"Right now, it's very hard to get any corporate investment," said James D. Murray, a professor at the University of California, Davis, who developed the goats with the infection-fighting milk. "What studies do you need to do? What are they looking for?" he said, referring to government regulators. "That stuff's not there."
But some experts caution that even if the F.D.A. clears the regulatory path in coming months, investors and agribusiness companies might still shy away. Many fear that consumers would shun foods from transgenic animals, sometimes referred to as genetically modified organisms, or G.M.O.'s.
"The companies we have spoken to have gone organic, and they are very concerned, at least up to the present time, of having G.M.O. associated with their name," said Cecil W. Forsberg, a professor at the University of Guelph in Ontario, Canada, who helped developed the "Enviropig" with the cleaner manure. Smithfield Foods, for one, the world's largest hog producer and pork processor, says it is doing no research on genetically engineered animals.
Critics say changing the genes of animals could lead to potentially harmful changes in the composition of milk or meat, like the introduction of a protein that could cause allergic reactions. They say there could also be risks to the environment if, for example, extra-large salmon were to escape into oceans and out-compete wild salmon for food or mates. Some also say that some of the processes used to create transgenic livestock can harm the animals themselves.
The federal guidelines would come after more than 15 years of talks and false starts at the F.D.A., a delay irking not only developers of the transgenic animals but also critics of biotechnology.
"The fact that the agency has sat there for years staring this problem in the face and really hasn't come up with a clear way to regulate this is abdicating its responsibilities," said Joseph Mendelson, the legal director of the Center for Food Safety, a Washington advocacy group.
Even now, the F.D.A. will not say when the rules will be ready.
"We want to get it out, but we also want to get it right," said Julie Zawisza, a spokeswoman for the agency, which declined to make any other officials available for comment.
Some industry executives and former and current government officials say one reason for the delay was that some government officials, in part because of a preference for fewer regulations, wanted less stringent rules than the F.D.A. is considering.
Meanwhile, the biotechnology industry is actually pushing for the tougher standards.
"Our overarching goal is to have public confidence in our products," said Barbara Glenn, the managing director for animal issues at the Biotechnology Industry Organization, a trade group. "We won't have that unless we have a very strong review process."
The F.D.A. is turning to transgenic animals after having tentatively declared in December that milk and meat from livestock that is cloned — but not otherwise genetically manipulated — was safe for people to eat.
The F.D.A. considers clones to be less biologically radical than genetically engineered animals — which instead of being mere replicas of naturally occurring animals are given foreign DNA, usually from another species.
Larisa Rudenko, a senior biotechnology adviser in the F.D.A.'s veterinary drug division, said in a May presentation at the biotechnology industry's annual convention that each new type of genetically engineered animal would require approval for use in the food supply. That will be done, she said, under the umbrella of existing rules for drugs used in treating animal diseases.
While the implanted gene is somewhat like a drug, the existing rules would have to be stretched to fit.
But industry executives and some former agency officials said it was unlikely that Congress would enact totally new laws for transgenic animals. And using the drug laws, they say, would provide tighter control than an alternative approach of using the rules governing food additives. Agency officials have said that the veterinary drug rules would be used, and they have already been overseeing some experimental work on that basis. But they continued to debate the issue, and the policy has never been made official.
The regulatory guidelines would indicate how the drug rules would be interpreted for transgenic animals, and what types of data would be needed to prove safety and efficacy. But there are open questions about how the drug rules would actually translate. While a chemical drug must be shown to be consistent and stable, for instance, it is unclear how that standard would apply to a gene passed from generation to generation. Some critics say that while the drug rules do provide fairly strict regulation of food safety, there are drawbacks to adapting that approach. Because applications for approval of drugs are confidential, for instance, there would be no opportunity for public comment before the agency acted.
"In order to create confidence in a new technology, you really don't want behind-closed-door proceedings," said Margaret Mellon, director of the food and environment program at the Union of Concerned Scientists.
Another worry is that the F.D.A. might not have enough expertise or authority to conduct a vigorous review of the environmental impact of transgenic animals. The F.D.A. has dismissed this concern, however, saying it has sufficient expertise and can consult with other agencies.
The biotechnology regulatory branch of the Department of Agriculture created an animal division last December to figure out what its role should be.
Genetically engineered animals are often created by injecting the gene of interest into a single-cell embryo. A more recent technique that is more efficient is to put the gene into a skin cell and create an embryo from that cell by cloning.
In both cases, the embryo with the foreign gene is then implanted into the womb of a surrogate mother. After some transgenic animals are born, additional ones can be made by conventional breeding, because the foreign gene generally will be passed on to some of the offspring, as would any other gene.
The fast-growing salmon is the transgenic animal that has been swimming upstream the longest at the F.D.A. Its developer, Aqua Bounty Technologies of Waltham, Mass., has been working to win agency approval for about 10 years. Aqua Bounty's fish are Atlantic salmon that have been given a growth-hormone gene from the Chinook salmon. They have also been equipped with a genetic on-switch from a fish called the ocean pout, a distant cousin of the salmon.
Normally, salmon produce growth hormone only in warmer months, but the pout gene's on-switch keeps the hormone flowing year round. That enables the Aqua Bounty fish to grow faster, reaching their market weight in about 18 months instead of 30.
Elliot Entis, Aqua Bounty's chief executive, said the company had already given the F.D.A. studies showing that the fish were healthy and that the implanted gene remained stable over generations.
He said the company also had tests done to show that its fish contained the same level of fats, proteins and other nutrients as other farmed salmon and would not set off unexpected allergic reactions in people who eat them. The fish also taste the same as other farmed Atlantic salmon, Mr. Entis said.
"Nobody has ever analyzed salmon as closely as we have had done," he said. But the F.D.A. is asking for more data on safety and potential environmental effects on wild salmon.
Industry executives say the Enviropigs would be the next candidate for F.D.A. approval. The pigs contain a bacterial gene that allows them to produce an enzyme that helps them more fully digest a vital nutrient, phosphorus, in their feed. That means less phosphorus in the manure, which in turn could mean less phosphorus running off into lakes and oceans, where it can cause algal blooms and fish kills.
MaRS Landing, a technology promoting organization in Ontario, is trying to find a corporate partner for the pig, said John Kelly, the agency's executive director.
Less far along in the approval pipeline are pigs that contain a gene from the roundworm allowing them to produce omega-3 fatty acids, a nutrient normally found in fish that is good for the heart. That, in theory, could make eating pork or bacon healthier, although that has yet to be tested.
Jing X. Kang, an associate professor at Harvard Medical School who helped direct the project, said the researchers were looking for corporate backers while also trying to raise the level of omega-3 in the meat.
Carol Tucker Foreman, director of the Food Policy Institute at the Consumer Federation of America, a consumer advocacy group in Washington, said regulations might not assuage consumers, many of whom object to the genetic engineering of animals on humane or ethical grounds, more than on safety concerns.
"The fact that the F.D.A. has a powerful regulatory process for reviewing genetically engineered animals, far greater than they apply to genetically engineered crops, may not make any difference at all," Ms. Foreman said. "Because that's not what it's all about."