The ultimate goal of Mayo Clinic is to provide the highest standard of patient care – and that includes patients who volunteer to participate in clinical research trials. Before permission is granted to conduct a clinical trial, investigators must provide evidence to show that the potential drug or therapy being studied has a good chance of being safe and effective. They do that based on data produced from studying an animal model.

An animal model is a living organism that closely resembles the same health problem occurring in humans. The mouse is the most common animal studied in research because it is biologically similar to humans, produces multiple offspring, has a short reproduction cycle, develops rapidly, and is cost-effective. The mouse may have inherited the health problem to be studied or have acquired it naturally. However, in many cases, the pathological process needs to be induced.

When researchers knock out the function of one or more of a mouse's genes, it's called a knockout mouse. They begin with cultured cells grown in the laboratory. First, they insert artificial DNA into the chromosomes of embryonic stem cells harvested from mouse embryos. Then they inject the altered cells into mouse embryos, which they implant into the uterus of a female mouse. Knockouts are produced when her offspring are crossbred with other genetically engineered mice; they need a copy of the gene on each chromosome. Knocking out the activity of a gene and observing the appearance and behavior of the knockout increases the understanding of a gene's involvement in human disease.

When researchers transfer genes or DNA into a mouse's cells, it's called a transgenic mouse. The process involves introducing a cloned gene into the mouse genome and requires the breeding of 10 generations to consistently produce mice with extra copies or altered copies of a gene. Transgenic mice are critical for studying genetic function and for testing drugs.

— Yvonne Hubmayr, March 2010