Mammalian cloning: possibilities and threats

The cloning of mammals originated with the production of limited numbers of genetically identical offspring by blastomere separation or embryo splitti ng. In the past few years, remarkable progress has been reported in cloning by nuclear transfer (NT) with donor nuclei recovered from embryonic, fetal or adult cells. Factors that contribute to the successful reprogramming of the transferred nucleus and the normal term development of the newly recon structed embryo include the cell cycle stage of both the donor nucleus and recipient cytoplast, the timing of fusion and cytoplast activation, and the source of donor nuclei. The possibility of producing live offspring by som atic cell NT carries potential applications in animal husbandry, biotechnol ogy, transgenic and pharmaceutical production, biomedical research, and the preservation of endangered species. However, the low efficiencies of cloni ng by NT coupled with high embryonic, fetal and neonatal losses may restric t immediate commercial applications in agriculture. These limitations notwi thstanding, the greatest benefits and practical implications of this new te chnology could be in transplantation medicine and therapeutic cloning.