To clone a living thing is to make an exact genetic copy of that organism. Individual genes—the biochemical building blocks that govern the structure and function of all living creatures—can be cloned, as can whole cells. Both gene and cell cloning are common research tools in current genetic and biomedical research.

Entire organisms can also be cloned. For example, humans have cloned plants for centuries by use of small cuttings—a process called vegetative propagation. Some invertebrate animals—starfish and earthworms, for example,—grow into two identical organisms when split apart, but most animals differ from plants in that they cannot be cloned so readily. In the 1980s scientists began researching methods of cloning higherorder animals—mammals in particular. The accelerating success of their experiments has led to widespread discussion over the possibility of human cloning. This discussion has revealed...