The production of transgenic domestic livestock: successes, failures and the need for nuclear transfer.

The direct transfer of recombinant DNA to embryos is conceptually a powerful method for the manipulation of the genetic potential of domestic animals, but in practice the technology has yet to fulfil its promise. In this paper, two examples are given of research utilising direct genetic manipulation, both of which are aimed at increasing aspects of productivity in sheep. The first of these involves the modification of the growth hormone status of sheep by the use of the ovine growth hormone gene, the regulation of which has been altered by the use of an ovine metallothionein promoter. While there has been a large amount of research already conducted in this area by many groups, our recent results provide one of the first demonstrations that this approach can increase the growth rates of sheep while maintaining the animals in good health. The second project involves the modification of intermediary metabolism in sheep by the introduction of the cysteine biosynthetic pathway. The results to date demonstrate that it is possible to change intermediary metabolism in animals using our approach but that there are species-specific requirements that must be satisfied in order to make the approach a practical method for improving animal productivity.