The consequences of a gene knockout tell us about the function of that gene. Conditional changes, which can be activated at specific time points or in selected tissues, help in establishing the gene’s function at a specific age, or in specific cell types.

It is also possible to introduce precise changes into the protein coding part of a gene. This can be done with the purpose of mimicking a human mutation believed to cause disease. Alternatively, a mouse could be made to produce the human version of a protein. Such studies improve our possibilities for studying human disease mechanisms and for developing and testing new pharmaceuticals.

The genome sequencing projects have taught us that mammals have 22–23,000 genes, more than 90% of which have a function shared between mouse and man.

To date more than 10,000 genes have been targeted in mice. More than 500 different models of human diseases have been produced by gene targeting, including models for hypertension, atherosclerosis, cancer, diabetes and cystic fibrosis.