Transgenic animals

Transgenic animals have had their DNA manipulated so that they carry and express an extra, foreign gene.

Rats, rabbits, pigs, sheep, cows and fish have all been made transgenic, yet mice make up over 95 per cent of the transgenic animals that exist.

Normal physiology and development: they reveal how genes are regulated and how they affect body function and development, for example the role of insulin-like growth factor, studied by adding genes from other species that alter its formation.

Study of disease: they serve as models of human diseases, so new treatments can be investigated. Transgenic models exist for cancer, cystic fibrosis, rheumatoid arthritis and Alzheimer's disease.

Biological products: animals given the gene for a product can make costly medicines, such as human α-1-antitrypsin, a treatment for emphysema. Work along the same lines targets phenylketonuria (PKU) and cystic fibrosis.

In 1997 Rosie, the first transgenic cow, produced milk enriched with human protein (2.4 grams per litre). It contained human alpha-lactalbumin and was nutritionally more balanced for human babies than natural cow milk.

Vaccine safety: transgenic mice now check that the polio vaccine is safe before it reaches humans. If reliable, they could replace monkeys in testing vaccine batches.

Chemical safety (toxicity) testing: animals carrying genes that make them more sensitive to toxic substances than normal animals are exposed to the substances, giving results in less time.