Transgenic Animals

A transgenic animal is an animal that has had a foreign gene -- often called a transgene -- deliberately and permanently introduced into its genome, usually by injecting the foreign DNA directly into a fertilised egg cell or a very early embryo, such that the transgene becomes integrated into essentially every cell of the resulting animal, including its reproductive cells, allowing the introduced trait to be passed on to the animal's own offspring in turn. Transgenic animals -- mice being by far the most commonly used, owing to their small size, short breeding cycle, and well-studied genetics -- are produced for several distinct and valuable purposes.

One major purpose is studying how a particular gene functions and, in particular, how a malfunctioning version of a gene contributes to a disease. By introducing a specific human gene associated with a disease -- for instance, an oncogene linked to a particular type of cancer -- into a mouse embryo, researchers can create a transgenic mouse strain that reliably develops that disease, or shows measurable symptoms related to it, providing a living model organism in which the disease process can be studied in detail and in which potential treatments can be tested, in ways that would be neither practical nor ethical to study directly in human patients.

A second major purpose is using a transgenic animal as a 'bioreactor' -- essentially, a living factory for producing a valuable protein, most often a protein of pharmaceutical or therapeutic value for humans. This is achieved by introducing the gene for the desired human protein linked to genetic control sequences that specifically direct the transgene to be expressed only in the animal's mammary gland tissue, so that the desired protein is secreted directly into the animal's milk, from which it can then be extracted and purified relatively easily, without disturbing the animal's normal health or physiology. Transgenic cattle, goats, and sheep have all been engineered in this way to produce specific human proteins -- such as particular blood-clotting factors, or other therapeutic proteins -- in their milk, offering a production method that can, in some cases, be more practical or economical at scale than producing the same protein in bacterial or cell cultures. …