1. Whitehead Institute for Biomedical Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA
2. Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA
3. University of Vienna, A-1010 Vienna, Austria

Correspondence to: Rudolf Jaenisch^1,2 Correspondence and requests for material should be addressed to RJ (e-mail: Email: jaenisch@wi.mit.edu).

Cloning from somatic cells is inefficient, with most clones dying during gestation^{1, 2}. Cloning from embryonic stem (ES) cells is much more effective, suggesting that the nucleus of an embryonic cell is easier to reprogram^{3, 4, 5, 6, 7}. It is thus possible that most surviving clones are, in fact, derived from the nuclei of rare somatic stem cells present in adult tissues, rather than from the nuclei of differentiated cells, as has been assumed^{1, 8, 9}. Here we report the generation of monoclonal mice by nuclear transfer from mature lymphocytes. In a modified two-step cloning procedure, we established ES cells from cloned blastocysts and injected them into tetraploid blastocysts to generate mice. In this approach, the embryo is derived from the ES cells and the extra-embryonic tissues from the tetraploid host⁶. Animals cloned from a B-cell nucleus were viable and carried fully rearranged immunoglobulin alleles in all tissues. Similarly, a mouse cloned from a T-cell nucleus carried rearranged T-cell-receptor genes in all tissues. This is an unequivocal demonstration that a terminally differentiated cell can be reprogrammed to produce an adult cloned animal.