For an animal to serve as a research model, it must have several characteristics: It must be relatively small and easy to keep. It must also reproduce rapidly. Only if these requirements are met can reliable scientific results be obtained with reasonable resources and within an acceptably short time.
The mouse meets all these requirements. With a life expectancy of two to three years, the complete life cycle of the mouse can be studied far more quickly than that of humans, for example. After a short gestation period of just three weeks, three to ten offspring are born. Mice also mate with close relatives. In this way, inbred lines can be produced that are genetically very similar. This is important for scientists, as it enables them to obtain precise, comparable experimental results.
Moreover, for a long time the mouse was the only species in which researchers were able to switch off individual genes. Only in recent years have new techniques enabled scientists to do this in other species as well. Few mammals have therefore been as extensively studied as the mouse. Its genome has been full sequenced. A comparison of its genes with those of humans shows that despite their dissimilar appearance, mice are biologically very similar to humans: 95 percent of the genes in the mouse genome also occur in similar form in humans. Many of the diseases of mice and humans have the same genetic cause.
Researchers can switch genes on and off in the genome of the mouse precisely and at specific times in the development process and then study the effects. This allows conclusions to be drawn about the function of a gene. Mice with an altered variant of a gene often show similar tissue changes as human patients. This allows scientists to clarify the emergence of diseases in humans and their treatment. In 2015, 55 percent of the mice kept at Max Planck Institutes were genetically modified.
In many cases, the genetically modified animals do not exhibit any noticeable differences. This is the case, for example, when a gene defect is compensated for by other genes. Such genetic modifications have little or no effect on the animal and are only detectable via special methods, if at all.
Only rarely do gene modifications trigger disease symptoms: in 2015 just three percent of the mice at Max Planck Institutes had disease symptoms due to an intentionally induced mutation.
In principle, the scientists are obliged by law to keep the number of test animals with health-impairing genetic modifications as small as possible. They can minimize the number of such animals in various ways, for example, by breeding heterozygous animals, i.e., animals with only one pathogenic version of a gene, if the modification only affects animals with two copies of the defective gene.
If a genetic modification is expected to cause distress, the breeding of such animals in itself is classified as an animal research - irrespective of the studies to be carried out on the animal later. Scientists in Germany must therefore carry out a distress assessment for any genetically modified strain. In the case of expected or unexpected distress, the breeding and keeping of that strain must be approved by the authorities.