Differentiated mammalian somatic cell nuclei and embryonic nuclei can now be reprogrammed to develop into young if they are introduced into enucleated oocytes. The success rates for cloning are generally low, however, and peri- and postnatal death rates of the young are high. Cloning technology will be useful for the genetic improvement of farm animals, therapeutic human protein production, and organ or tissue transplantation into humans. In addition, the information obtained on nuclear reprogramming will be helpful for understanding the fundamental mechanisms of differentiation and aging.