A plant or animal that is genetically altered by the artificial introduction of foreign genes is transgenic, for genes have been transferred from one genome to another. Donor DNA is usually recombined in vitro with a vector before being introduced into recipient cells. The donor DNA may reside temporarily in the transgenic recipient, or it may become a permanent part of the recipient's genome, where it is stably inherited. Any species can be a recipient when workable methods of gene transfer are developed. It is also possible to inject mRNA into a recipient for transient expression there.
Transgenic organisms are useful for studying gene expression, phenotypic effects of mutations, and gene interactions as well as for discovering novel genes. Agricultural crop plants are modified genetically with the aim of improving the yield and quality of food and fiber. Gene therapy is the genetic modification of human cells to treat genetic disease; its enormous potential has yet to be realized.
Transient Gene Expression
Experiments with transient gene expression are useful for finding out what a protein does and determining the function of parts of the protein by inducing mutations in the gene that encodes it. The transgenic organism is a testing ground.
It often suffices to express a transgene or mRNA in vivo for a short time. This works if the foreign protein functions in transgenic cells and can be assayed quickly.