Since the early 20th century, when Hans Spemann first twinned salamander embryos, scientists have made astounding progress in the science and technology of cloning. They have now developed the means to apply cloning in research, agriculture, and medicine. With advancements in cloning techniques for stem cell research, scientists have been able to explore human diseases at a cellular level, attempting to better understand the cellular mechanisms involved in disease. Readers explore the history, science, applications, and ethical issues of cloning. Sidebars profile pioneers in the field, including John Bertrand Gurdon, Ian Wilmut, Shinya Yamanaka, and James Thomson.

The process of generating a living entity that is a genetic duplicate of another is known as cloning. The science and technology behind that process are powerful. They have the potential to treat human disease, to improve agriculture, and to advance scientists’ understanding of basic cellular processes.

The benefits of those advances to humankind could be immense. But the use of cloning technologies also raises significant ethical issues. Of particular concern has been the use of human embryos for the generation of cloned stem cells the cells that form the basis of therapeutic cloning and the potential of reproductive cloning to produce genetically identical copies of humans.

Therapeutic cloning and reproductive cloning are derived from more than a century of research and development. Reproductive cloning, in which the objective is to create an individual organism that is a genetic copy of another individual, generally is considered to have begun with the work of German embryologist Hans Spemann, who in the early 1900s artificially “twinned” salamanders. Spemann created twin salamanders by constricting salamander zygotes (fertilized eggs), such that in each egg, the cell nucleus was forced to one side, leaving the other half of the egg enucleated (without a nucleus). He let the zygote grow through several rounds of cell division before releasing the constriction to allow a single cell nucleus to slip into the enucleated side. He then closed off the constriction, cutting the embryo in two. The result was the generation of twin offspring. Spemann’s work laid the foundation for nuclear transfer, the process by which the nucleus of a cell is inserted into an enucleated egg cell. Nuclear transfer is at the centre of cloning technology.