Mammalian Cloning: Challenges for the Future

Publisher Summary Although the number of laboratories working on mammalian reproductive cloning has increased dramatically in the past few years, the proportion of eggs or zygotes with a genome that has been transferred from another cell remains obstinately at a few percent. Although most attrition occurs around the time of implantation, late-gestation and early-postpartum deaths, regardless of the species, are more prevalent in clones. An optimistic view would be that this is simply because of the limitations of existing techniques and that once these concerns have been addressed, higher success rates will be achieved routinely. Unlike in other species, reproductive cloning is not an option for critically testing the effectiveness of the reprogrammed donor nuclei in humans. Hence, for realizing therapeutic cloning in humans, an important outstanding issue is how to ensure that the differentiated progeny of embryonic stem (ES) cells derived in this way are safe for treating patients. In view of the persisting uncertainties and concerns, the opinion within the biomedical research community is divided as to whether therapeutic cloning will offer a viable approach for circumventing rejection in the harnessing of ES cells for tissue repair. The widespread present support for a moratorium on human reproductive cloning, both within the scientific community and outside it, is rooted in two types of concerns. Ethical considerations are dominant outside the scientific community, whereas concerns about safety have featured most prominently in the debate within the community. There are opposing views on whether worries about the safety of human reproductive cloning are warranted.