For almost two decades, introduction of new genes, transgenic technology, has been the source of many new strains of mice that have become valuable tools in various fields of research. More recently, knockout technology has generated a large number of mutant strains. To maintain all these lines and make them available to the scientific community, a strict breeding schedule must be followed. This can quickly become a financial burden, even for large institutions with adequate funding. Alternatively, cryopreservation offers the means to maintain mouse lines in the smallest space available, thereby reducing the requirements of time, energy, and funds. Such practices would involve freezing either mouse gametes (1 –8 ), where the lines would be revived through in vitro fertilization, or freezing mouse embryos where the line would be reestablished after recovering the embryos.
