Mammalian cloning by nuclear microinjection

In the cloning of whole animals, genetic information from a single cell programs the development of a new individual; it is a form of asexual reproduction. Consequently, cloning circumvents the requirement for a genetic contribution from two cells (spermatozoon and egg) exhibited in sexual reproduction. Cloning provides a method of producing multiple offspring from a single individual, embryo, or cell line, and cloned animals share much the same genetic relationship as identical twins. In practice, whole animal cloning (also referred to as “reproductive cloning”) is often defined euphemistically as a process in which the nuclear genome of a single cell programs full development; this is the definition we will adopt here. While the possibility that mammals might one day be cloned by nuclear transfer from adult cells has evoked enormous interest, some believed it impossible.