Developments in transgenic techniques in pigs

Manipulation of the pig genome is currently restricted to the random insertion of new DNA using pronuclear microinjection. This method suffers from a number of inherent limitations, the majority of which result from the inability to control the site at which the transgene becomes integrated. These drawbacks, together with the need to be able to target existing genes, will result in the replacement of pronuclear injection by new methods that have the capability to direct insertion to a particular genomic site that does not influence expression. Currently, it is possible to control the site of insertion in mice using embryonic stem (ES) cell and homologous recombination technologies. However, pluripotent ES cells have yet to be isolated in pigs. The possibility of using nuclear transfer to reprogramme early differentiated embryonic cells as well as somatic cells from adult animals may provide an alternative method for generating precise genetic modifications. Methods that allow these changes to be carried out in situ are also likely to be developed in the future.