In addition to the moral argument against the use of somatic-cell nuclear transfer for the creation of a child (“reproductive cloning”), there are overwhelming scientific reasons to oppose this practice. In contrast, many believe that the practice of somatic-cell nuclear transfer with the goal of generating an embryonic stem-cell line (sometimes referred to as “therapeutic cloning”) is justified, because it holds the promise of yielding new ways of studying and treating a number of diseases. Once isolated from a patient, an embryonic stem cell thus derived would be “customized” to the needs of the patient who had served as the nuclear donor and thus would obviate the need for immunosuppressive treatment as part of a therapeutic application. In addition, because embryonic stem cells can generate most, if not all, types of cells in vitro, a stem cell isolated from a patient with a complex genetic disease could be used to study the pathogenesis of the disease in culture. In Figure 1, the generation of a mouse by sexual reproduction is juxtaposed with its generation by nuclear cloning and the derivation of an embryonic stem-cell line by means of nuclear transfer from a patient’s cell.

In contrast to an embryo derived by in vitro fertilization, a cloned embryo has little, if any, potential to develop into a normal human being. By circumventing the normal processes of gametogenesis and fertilization, nuclear cloning prevents the proper reprogramming of the clone’s genome (described below), which is a prerequisite for the development of an embryo into a normal organism. It is unlikely that these biologic barriers to normal development can be overcome in the foreseeable future. However, embryonic stem cells derived from a cloned embryo are functionally indistinguishable from those that have been generated from embryos derived through in vitro fertilization. Both have an identical potential