Somatic cell nuclear transfer, technique in which the nucleus of a somatic (body) cell is transferred to the cytoplasm of an enucleated egg (an egg that has had its own nucleus removed). Once inside the egg, the somatic nucleus is reprogrammed by egg cytoplasmic factors to become a zygote (fertilized egg) nucleus.
The concept of mammalian clones, even humans, was not new at the time of Dolly’s birth. Among mammals, naturally occurring genetic clones, or individuals genetically identical to one another, had long been recognized in the form of monozygotic (identical) twins. Unlike Dolly, however, such clones are derived from a single zygote, or fertilized egg, and thus they are clones of one another, rather than clones of another individual. Moreover, clones had been generated previously in the laboratory, but only from embryonic cells that were either undifferentiated or only partially differentiated. In animals, the production of clones from fully differentiated (adult) cells (e.g., skin or muscle cells) had been carried out successfully only in lower species, such as frogs.