DNA (deoxyribonucleic acid) is considered the repository of an organism's genetic data. It serves as a form of "blueprint" for the various proteins that are produced in a cell and which essentially determine the properties of the cell. The DNA section that encodes a protein is referred to as a gene.

The DNA in transgenic animals has been deliberately modified: foreign genes are added ("knock-in") or "normal" genes are replaced by "non-functioning", i.e. non-readable, variants ("knock-out").

The modified DNA record can cause the transgenic animal to manifest itself differently (different phenotype) from other animals of the same species; for example, it might be short-lived or long-lived or it may carry a predisposition towards a particular form of cancer.

On the basis of the changes it is possible to draw inferences about the effect of the knocked-out or knocked-in gene. Through research with transgenic animals scientists hope to gain, inter alia, a deeper insight into the causes of genetically-related diseases. This knowledge is intended to assist with the development of innovative therapeutic procedures.

There are a number of reasons why mice are so often used for biotechnology studies: the relatively strong concordance of the mouse genome (the totality of genes is referred to as the genome) and the human genome, the short generation spans of the mouse and the animal's uncomplicated care all play a role. The mouse genome (like the human genome) has now been fully mapped.