Scientists in Britain have taken a significant step beyond Dolly by creating the first sheep clones whose genes have been "selectively modified".
The breakthrough means it may be possible to create cattle and sheep which are immune to diseases, notably BSE and scrapie.
This is predicted to lead to a new era in mammalian genetics including, in time, human genetics.
The team at the Edinburgh-based biotech company PPL Therapeutics was closely involved in the creation of Dolly the sheep, the first mammal clone created from an adult cell.
Accurate gene targeting has been carried out in laboratory mice for 15 years.
But until now it has only been possible to alter genes in higher mammals using a hit-or-miss method whereby substitute DNA randomly incorporates itself into the chromosomes.
A very high "miss" rate was the subject of controversy and had suggested the technology was unpredictable and ethically inappropriate.
The PPL team, however, succeeded in picking a specific chromosome region, inserting a new DNA gene sequence into it, and producing 14 clones all having the same modification.
