To clone humans is deliberately to generate two or more individuals who share the same nuclear deoxyribonucleic acid. Using animals, researchers have performed two basic types of cloning that will eventually yield commercial benefits. Embryo twinning involves separating the individual cells of an embryo and allowing each to cleave for later transfer to a uterus. Cloning by nuclear transfer involves removing the nuclei from embryonic cells or fetal or adult somatic cells and fusing those nuclei with enucleated donor egg cells. (...) Although for the purposes of commercial biotechnology these techniques do not raise undue concern when attempted with animals, embryo twinning, embryo nuclear transfer, fetal somatic cell nuclear transfer, and adult somatic cell nuclear transfer all provoke intense ethical discussions when considered for human use. (shrink)