The first successful cloning experiments in vertebrates arose from the desire of embryologists to know whether the process of cell differentiation from an egg involved permanent or stable changes in the genome. One idea was that, as cells differentiate, the genes no longer needed (such as skin genes in intestinal cells) could be lost or permanently repressed. The other idea was that all genes are present in all cell types, and that cell differentiation involved the selective activation and repression of genes appropriate to the cell type. The transfer of nuclei from differentiated cells to an egg could answer these important questions. In 1952, the first successful transplantation of nuclei from early embryo cells was achieved with the American frog Rana pipiens.
In the late 1980s, scientists took cloning to the next stage by cloning other mammals (cattle, sheep, pigs, mice, and rhesus monkeys). This is seen as the first step toward the cloning of mammals closest to humans. But they were limited in their success to using the early unspecialized embryo cells, i.e., not adult cells. All this background work led to Dolly, the first mammal to develop from the nucleus of an adult somatic cell.
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