By Rachael Buchanan
BBC News, in Boston
Imagine you could get life-saving medicines from milking a common farmyard animal.
Down on the "pharm": The goats of Massachusetts
That idea moves a step closer to becoming a reality this week, as the European Medicines Agency (EMEA) considers the final stages of an application to license a natural human protein extracted from the milk of goats.
If the EMEA says "yes" on Thursday, Atryn will become the world's first medicine to be produced from a genetically modified animal and represents the vanguard of this long-promised science.
The 57 unique goats that produced the drug-laden milk reside on an anonymous-looking farm belonging to GTC Biotherapeutics, an hour's drive from Boston, Massachusetts.
To the eye, they are indistinguishable from their fellow ruminants, jostling in their pens for a better look at their visitors. But what marks them out is an extra snippet of DNA entwined in their genome.
It is a human gene that codes for anti-thrombin (AT), an anticoagulant - a substance that inhibits blood clots from forming. It is usually extracted from blood plasma.
A decade and a half ago, the company's scientists copied the human AT gene and attached it to a chunk of goat DNA, the promoter for beta casein. This gives the instruction to express that gene only in milk and no where else in the body.
This genetic composite was then injected into a newly fertilised goat egg and as the sperm and egg fused together, the extra gene was incorporated into the goat genome.
The embryo was then implanted into a surrogate mother and six months later the herd founder was born.
For GTC's British born CEO, Geoffrey Cox, goats are an obvious choice of bioreactor.
"It takes just 18 months to produce a lactating animal and in a single year one goat produces the equivalent of 90,000 blood collections," he told the BBC News website.
Potential recipients of Atryn number around 1 in 5,000 in the UK.
Sufferers are born missing one copy of the anti-thrombin gene, resulting in underproduction of this protein and leaving them prone to blood clots.
Normally patients are maintained on blood thinners such as Warfarin but if they are giving birth or undergoing surgery that is too risky, and they are given replacement anti-thrombin.
The only current source is human blood plasma. While there have never been any contamination problems with AT products, fears about the possible transmission of diseases such as vCJD, as has been seen with whole blood, make doctors unwilling to expose their patients to plasma products unless they have no choice.
The key alteration takes place in a newly fertilised goat egg
Hayley Jarvis is one such patient - having AT deficiency meant taking plasma derived anti-thrombin when she had son Oliver.
In a month's time she will take the drug again when she delivers her second son.
Hayley says she wouldn't have a problem taking a medicine from a goat. She knows any risks from plasma are theoretical but "there is always that niggle at the back of your mind," she says. "It would be good to have a choice."
Her physician, Dr Beverley Hunt of St Thomas' hospital London, is an expert in thrombosis.
She agrees and would welcome an alternative, but Dr Hunt is not put off by the animal origin of the drug.
"Yes we do need to think about risk of disease transmission from animals, but we already use direct animal products such as heparin from pig tissue and leech extracts for blood thinning, without causing harm to people".
GTC's Atryn goats are not alone in this novel science. Mixed in with the company's 1,500 strong herd are goats producing a treatment to shrink solid tumours.
Down the road in Wisconsin, Dutch firm Pharming keep a herd of cows expressing human lactoferrin - a protein found in breast milk which has anti-bacterial qualities.
Three and a half thousand miles away on home turf in the Netherlands, Pharming are milking rabbits for a treatment for hereditary angioedema, which leads to swelling in various parts of the body.
Life-saving goats, cows, rabbits - it's a long way from the laboratory mice that were the first living transgenic drug factories in 1987.
Back then, human therapeutic proteins produced in animal milk appeared to offer great economic potential.
In the late 1990s, biotech investors went wild for the promise of living, breeding drug producers, delivering products at a fraction of the cost of a traditional biotech factory. But progress has been slow.
Milking future: Goats offer the possibility of high yields
It is nearly two decades since that pioneering mouse and 14 years since the birth of GTC's first Atryn goat, and not a single product has made it to market yet.
As investors lost their patience, companies have faced leaner times and some have gone to the wall.
Now Atryn stands on the brink of European licensing and Pharming's Lactoferrin could get FDA approval later this year. Maybe 2006 will be the year transgenic drug production finally makes the leap from bench to bedside.