The chicken egg has a storied history in medicine. Even today, millions of ordinary fertilized eggs are each punctured with a drill and injected with flu virus to make vaccines. Now, scientists at the same research institute that cloned Dolly the sheep have produced a genetically modified rooster whose female descendants lay eggs that produce medicines in place of a protein in egg whites.

Helen Sang of the Roslin Institute in Edinburgh, Scotland, and her colleagues used lentivirus to introduce a gene into freshly fertilized chicken embryos that trigger the production of various drugs rather than the protein ovalbumin, which normally makes up roughly 54 percent of egg whites. The researchers screened the resultant cockerels for one that produced the new gene in its semen. They then bred him with normal hens to produce a flock of chickens that carried the inserted gene thereby producing medicines in their egg's whites.

Tests of the flocks' eggs showed that they could produce either miR24--a monoclonal antibody used in treating melanoma--or interferon b-1a--an immune system protein used against multiple sclerosis, among other things--depending on which gene was inserted. The chickens produce 15 to 50 micrograms per milliliter of egg white, the researchers found, and though this is not as efficient as the expression of ovalbumin, it is efficient enough to allow for subsequent purification into therapeutic drugs. "We would expect the transgene not to be as efficient as the endogenous gene it was based on as only some of the regulatory elements were used and the transgene may be inserted in the chromosome at a position that does not favor anywhere near maximal expression," notes Roslin's Adrian Sherman, who also participated in the research. "I'm sure there is potential for improvement."

Chicken eggs may prove a better way to producepharmaceuticals than other genetically modified products (such as goat milk) that have been previously explored. Chickens are easy to raise, produce numerous eggs, and are cheap to keep. And, after raising five generations of the modified birds, the researchers have observed no adverse health effects, according to the paper published online January 15 in Proceedings of the National Academy of Sciences USA.

Even though the therapeutic proteins worked as intended during in vitro assays, it will be years before the process is ready to be used to produce drugs for human consumption, researchers say. Roslin's chickens join a similar effort using stem cells developed by Origen Therapeutics. Regardless of which "biofactory" delivers drugs first, a new medicinal use for the venerable egg is now apparent.