Attempts to obtain eggs and offspring using oocytes in ovary have been made for keeping desirable traits in livestock, preserving human fertility, etc. Ovarian transplantation, one of those methods, has been established and widely used. Although transplantation to different species is expected to bring benefits, it is still too difficult to produce offspring. Researchers have developed a novel system using xenotransplantation (transplantation of organs from one species to different one), and succeeded in producing offspring.