Nuclear transfer experiments in mammals have shown that the nucleus of an adult cell has the ability to direct the development of an entire organism, id est its genome is totipotent. However, these experiments did not conclusively demonstrate that the nuclei of terminally differentiated adult cells remain totipotent. It is possible that rare adult stem cells served as donors for the few surviving clones. To address this question, we have generated monoclonal mice from terminally differentiated lymphocytes that carry a single antigen receptor rearrangement in all tissues. Nuclear transfer technology may provide a powerful method for obtaining autologous cells for replacement therapy. We have demonstrated the feasibility of this concept by combining nuclear transfer with gene and cell therapy to treat the immune deficiency of Rag2 mutant mice, thus establishing a paradigm for 'therapeutic cloning'. Moreover, we will discuss the potential use of nuclear transfer to study the role of reversible genomic (epigenetic) modifications during tumorigenesis.