Animal experiments need numerous kinds of animal which are suitable for every research. About 300 mouse strains are developed up to the present, but they do not give satisfaction to every researchers. So we must build up the methods of breading animals which are newly developed and of maintenance of characteristics which were developed before. To maintain experimental animal is not only proceeding the generation but also increasing the animal populations, it needs geneticai control. Genetic factors which influence to reproduction are very important to maintain colony. These factors include lethal gene, chromosomal abberation, sterility gene, etc.. With the recent development of transgenic technology, scientists now can deliberately creat numerous specific animal models. To know how to manage the colony which has genetic defect on reproduction and transgenic mice is one of the key to study in vitro fertilization.