UK scientists have developed genetically modified chickens capable of laying eggs containing proteins needed to make cancer-fighting drugs.
GM chickens could be a route to faster, cheaper drugs
The breakthrough has been announced by the same research centre that created the cloned sheep, Dolly.
The Roslin Institute, near Edinburgh, says it has produced five generations of birds that can produce useful levels of life-saving proteins in egg whites.
The work could lead to a range of drugs that are cheaper and easier to make.
Professor Harry Griffin, director of the institute, told the BBC: "One of the characteristics of lots of medical treatments these days is that they're very expensive.
"The idea of producing the proteins involved in treatments in flocks of laying hens means they can produce in bulk, they can produce cheaply and indeed the raw material for this production system is quite literally chicken feed."
Roslin has bred some 500 modified birds. Their existence is the result of more than 15 years' work by the lead scientist on the project, Dr Helen Sang.
But it could be another five years before patient trials get the go-ahead and 10 years until a medicine is fully developed, the Roslin Institute cautioned.
Therapeutic proteins such as insulin have long been produced in bacteria; but there are some complex proteins that can only be made in the more sophisticated cells of larger organisms.
Scientists have successfully made a range of these molecules in the milk of genetically modified sheep, goats, cows and rabbits.
The work at Roslin shows it is now possible to use chickens as "biofactories", too.
Some of the birds have been engineered to lay eggs that contain miR24, a type of antibody with potential for treating malignant melanoma, a form of skin cancer. Others produce human interferon b-1a, which can be used to stop viruses replicating in cells.
The proteins are secreted into the whites of the eggs. It is a fairly straightforward process then to extract and purify them.
Dr Sang said the team was highly encouraged by the level of the birds' productivity, but further improvements were required.
"We're probably getting a high enough productivity if you want to make a very active protein like interferon, but not enough yet if you want to make an antibody because people need large doses of these over long periods; so one of our next challenges is to try to increase the yield in egg white," she told BBC News.
Chickens had some advantages over other animals for "pharming" because their lifecycles were shorter, said Dr Sang.
"Once you've made the transgenic birds, then it's very easy; once you've got the gene in, then you can breed up hundreds of birds from one cockerel - because they can be bred with hundreds of hens and you can collect an egg a day and have hundreds of chicks in no time," she explained.
The Roslin research is part of the Avian Transgenic Project, a joint venture with biotechnology firms Viragen and Oxford BioMedica.
Details of the latest work are to be published this week in the US journal Proceedings of the National Academy of Sciences (PNAS).
The Roslin team also expects its engineered chickens to provide new insights into aspects of reproductive biology.
It says the ability to modify birds' embryos will allow researchers to study fundamental processes that control the very early development of vertebrates.
It is just over 10 years since the Finn Dorset lamb called Dolly was born at the institute.
She was the first mammal to be cloned from an adult cell - making her a genetic replica of a six-year-old ewe. She was put down in 2003 after contracting a common lung disease.