Challenges and Prospects for Targeted Transgenesis in Livestock

Until recently the only ways of making transgenic livestock were by pro-nuclear injection or, more controversially, by sperm mediated DNA transfer. Both techniques usually result in multiple copies of the transgene at a random site within the genome. This is associated with unpredictable transgene expression often due to gene silencing. In the mouse, this problem has been addressed by directing single copy transgenes to particular sites in the genome (gene targeting). This has been possible because of the availability, in that species, of embryo-derived stem cells (ES cells) which can be modifiedin vitroand then used to create transgenic mice. Despite considerable research effort, however, ES cells are not available for any livestock species. ES-like cells have been derived from sheep, pigs and cattle that can contribute to the formation of chimaeras but they do not contribute to the germline (reviewed by McWhir).