Human Cell 2006; 19: 2–10 doi: 10.1111/j.1749-0774.2005.00001.x © 2006 The Authors 2 Journal compilation © 2006 Japan Human Cell Society Blackwell Publishing Ltd REVIEW ARTICLE Cloned mice and embryonic stem cell establishment from adult somatic cell Satoshi KISHIGAMI, 1 Sayaka WAKAYAMA, 1,2 N VAN THUAN 1 and Teruhiko WAKAYAMA 1 1 Center for Developmental Biology RIKEN and 2 Department of Life Science, Graduate School of Science and Technology, Kobe University, Kobe, Japan Abstract Cloning methods are now well described and becoming routine. Yet the frequency at which cloned offspring are produced remains below 2% irrespective of nucleus donor species or cell type. Especially in the mouse, few laboratories can make clones from adult somatic cells, and most mouse strains never succeed to produce cloned mice. On the other hand, nuclear transfer can be used to generate embryonic stem (ntES) cell lines from a patient’s own somatic cells. We have shown that ntES cells can be generated relatively easily from a variety of mouse genotypes and cell types of both sexes, even though it may be more difficult to generate clones directly. Several reports have already demonstrated that ntES cells can be used in regenerative medicine in order to rescue immune deficient or infertile phenotypes. However, it is unclear whether ntES cells are identical to fertilized embryonic stem (ES) cells. In general, ntES cell techniques are expected to be applicable to regenerative medicine, however, these techniques can also be used for the preservation of the genetic resources of mouse strains instead of preserving such resources in embryos, oocytes or spermatozoa. This review seeks to describe the phenotype, application, and possible abnormalities of cloned mice and ntES cell lines. Key words: cloning, embryonic stem cell, nuclear transfer, reprogramming. INTRODUCTION Since it was first reported in 1997, 1 somatic cell cloning has been demonstrated in several mammalian species. While cloning efficiencies can range from 0 to 20%, rates of just 1–2% are typical (i.e. one or two live offspring are produced per one hundred initial embryos). Recently, abnormalities in mice cloned from somatic cells have been reported, such as abnormal gene expression in embryos, 2,3 abnormal placenta, 4 obesity or early death. 5–7 Such abnormalities notwithstanding, success in generating cloned offspring has opened new avenues of investigation. It also provides a valuable tool that basic research scientists have employed to study complex processes, such as genomic reprogramming, imprinting and embryonic development. A number of potential agricultural and clinical applications are also being explored, including therapeutic cloning for human cells, tissues and organ replacement, and the reproductive cloning of farm animals. Another such application is the preservation of the genetic resources of mouse strains. Before the success of mouse cloning, the maintenance of mutant mice with severe sterility, such as those without germ cells, was impossible even if they showed a number of interesting phenotypes. Unfortunately, the current success rate of mouse cloning from adult somatic cells remains very low; usually less than 2% of the cloned embryos develop to term. 4,8 Moreover, the success rate depends on the mouse strain. 9,10 Usually, a hybrid strain is more suitable for mouse cloning than an inbred strain. C57BL/6 and C3H/He are popular mouse strains in mouse genetics but have Correspondence: Dr Teruhiko Wakayama, Center for Developmental Biology RIKEN Kobe, 2-2-3 Minatojima-minamimachi, Kobe 650-0047, Japan. Email: teru@cdb.riken.jp Received 1 November 2005; accepted 11 November 2005.