The presence of any type of mouse pathogen (whether it causes clinical disease or not), changes the physiology of the mouse, which can lead to false experimental results. Mouse strains harboring pathogens can be ‘rederived’ to a pathogen-free status by transferring pre-implantation embryos from a contaminated mouse to a pathogen-free foster mother. The resulting pups will then also be pathogen-free.
The purpose of cryopreservation is to protect valuable, unique mouse lines against loss through breeding failure or disease, and to eliminate the cost of maintaining mouse lines that are not in use. Sperm freezing is the principal method as it is inexpensive, quick and requires only a small number of mice. In vitro fertilization (IVF) with the frozen sperm is required to revive the mouse line. For complex genetic backgrounds, embryo freezing is the preferred method to archive the mouse line.