Commentary on human cloning.

Human cloning refers to the production of genetically identical humans. This cloning is possible via either ‘‘embryo splitting’’ or ‘‘nuclear transfer’’. Embryo splitting involves the separation of an early human embryo into two or more parts. Each of these parts has the potential to develop into a blastocyst (late embryo), which, if implanted, can develop into a child. This is how genetically identical monozygotic twins are created. Artificial embryo splitting has been successfully implemented in various mammals including sheep (Willadsen, 1981), cows (Willadsen, 1989), mice (Agrawal and Polge, 1989) and monkeys (Chan et al., 2000), but has been performed only to the pre-implantation stages in humans (Hall et al., 1993). Recently the American Society for Reproductive Medicine declared that human cloning by artificial embryo splitting was an ethical procedure to increase the number of implantable human blastocysts used in certain infertility treatments (ASRM, 2000). However, embryo splitting can produce only a limited number of cloned individuals as the early embryo can be separated only a limited number of times, and the procedure is not able to produce a ‘‘clone’’ of an adult that already exists. The other method for producing cloned humans, nuclear transfer, does not suffer from these limitations, and the rest of this article will focus on human cloning achieved via this technique. Nuclear transfer (or more specifically somatic cell nuclear transfer) is a conceptually simple procedure. The