Scientists at PPL Therapeutics have used gene targeting to produce cloned sheep with a precise genetic alteration. The UK biotech firm, which is the commercial offshoot of the institute that cloned Dolly the sheep, published their results in last week's Nature.

Gene targeting allows researchers to add, replace or remove specific genes to produce genetically altered (transgenic) animals. Transgenic mice have been produced in this way for the last 15 years, but this is the first time the technique has been successfully used in another mammal. Other methods used to produce transgenic animals can only add genes randomly into the genetic code of the animal, with unpredictable results.

By combining gene targeting with cloning technology, PPL have created two cloned lambs that have the gene for a human protein precisely inserted into their own genetic code. As a result, Cupid and Diana will both produce human alpha-1 antitrypsin in their milk, a protein that could be used to treat cystic fibrosis and emphysema.

The achievement also has implications for scientists aiming to produce genetically altered pigs to use for human organ transplants, as it may now be possible to specifically knock out the pig genes responsible for organ rejection. The technique could also be used to develop more accurate animal models of human illnesses. In a commentary for Nature, Milind Suraokar and Allan Bradley said: 'We are clearly at the dawn of a new era in mammalian genetic technology'.