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1. The first studies producing genetically identical vertebrates by nuclear transfer were carried out in 1952 with frogs (Rana pipiens). Cloning of fish has also been feasible for many years and is now used in farming of fish for food. It was not until 1986 that the first cloned mammals (mice) were produced by transferring nuclear material from embryonic cells, and this was rapidly followed by successful nuclear transfer in sheep and, by 1990, in cattle. However, all these studies involved the transfer of nuclear material from embryonic cells which were considered to be totipotent, i.e. capable of developing into the full range of tissue types.

2. Early in 1997, the revelation that the Roslin Institute in Edinburgh had produced a cloned sheep (named Dolly) by nuclear transfer of material derived from an adult cell received worldwide attention. This opened a debate on the welfare implications of cloning farm animals and on the wider ethical implications of both animal and human cloning. Previous reports on the cloning of sheep from fetal cells had received relatively little attention.

3. In considering cloning, it should be recognised that a range of technologies can be used to create several animals with identical genetic make-up. At the most basic, any procedure which tends to encourage the production of identical twins may be referred to as cloning. The key difference introduced by nuclear transfer is the speed by which an animal of one genotype can potentially be cloned into a large number of other animals of identical genotypes. Another consideration of nuclear transfer is that it affords the opportunity to alter the genes within the nucleus prior to the transfer process, and to select the most optimally altered cells for multiplication. This might allow very specific alterations (gene targeting) to be carried out, and the selected genotype to be rapidly multiplied.