The presence of any type of pathogen (whether it causes clinical disease or not), may change the physiology of the mouse, which can lead to false experimental results. The process of rederivation eliminates pathogens such as pinworm, parvovirus, and mouse hepatitis virus from infected mouse lines. The investigator is responsible for providing mice of the strain in question. Pre-implantation embryos, which are relatively resistant to such pathogenic microorganisms due to the physical barrier of the zona pellucida surrounding the embryo, are collected from infected mice, washed through several droplets of embryo culture media, then surgically transferred into specific pathogen-free (SPF) pseudo-pregnant females . The mice will be tested to verify the SPF status of the offspring. This method eliminates viral (e.g. murine hepatitis virus, Sendai virus, parvovirus), bacterial (e.g. mycoplasma pulmonis), ectoparasite (mites), and endoparasist (pinworm) infections. Resulting mice can then be transferred to the barrier-level vivarium. Rederivation allows the import of established mouse lines with either known pathogens or of unknown health status from anywhere in the world.