Animals can be genetically modified to produce substances that are useful for treating human diseases. Animals that have been given a gene from another species are called transgenic animals.
The human gene that codes for the required protein is isolated from the human cells and is then injected into the fertilised egg from the animal.
The tiny embryos that develop are placed in the womb of surrogate mothers, which later give birth to young that carry the human gene in their cells.
Transgenic sheep are used to produce alpha-1-antitrypsin (AAT), a protein that is used to treat emphysema in people who have a mutation in their AAT gene. The sheep secrete the AAT in their milk.