Today, "Take the white of an egg... " is a recipe. Tomorrow, it could be a prescription. Scientists have discovered how to make hens lay eggs with drugs inside.

Researchers have long brooded over how to create genetically modified hens which would produce medicinal proteins in eggs. Now they have cracked it.

A team from a US company, AviGenics, reports in Nature Biotechnology that it managed to insert a foreign gene into the DNA of white leghorn chicken embryos. When the chickens matured, they laid eggs containing the protein which the gene gave the code for.

Levels of the alien protein remained steady in egg after egg, and in the eggs of chickens reared from the original eggs.

The technique will not be of use for producing traditional drugs such as aspirin. But it could produce large amounts of the increasing number of proteins which are being used as medicine.

Pharmaceutical omelettes and fried transgenic eggs for patients are not on the menu, however. The proteins will be extracted from egg whites, and the patients will see them in more conventional form.

Each egg is expected to yield about 17mg of medicinal protein or more, potentially enough for several doses.