Mammalian development has been thought to be an one way process, which starts with a few EMBRYONIC founder CELLS, that become more and more restricted and ultimately give rise to all specialized cell types of the body. The cloning of the sheep Dolly from an adult mammary gland cell has refuted this dogma and demonstrated that the developmental clock of a mature cell can be reset, or "reprogrammed" by the egg into that of an EMBRYONIC cell, which can support development of a copy of the donor animal. To exclude the possibility that adult STEM CELLS served as donors in successful cloning experiments, we have generated cloned mice from mature lymphocytes that carried immunoglobulin rearrangements in all tissues. This experiment demonstrated that even the nuclei of terminally differentiated adult CELLS remain competent to give rise to an entire cloned animal. In addition, reprogramming research has enormous therapeutic potential in humans as it may allow for the derivation of EMBRYONIC CELLS from patients’ CELLS suffering from degenerative disorders such as Alzheimer’s disease, Parkinson’s disease or diabetes; because EMBRYONIC CELLS have the ability to give rise to all cell types of the body when exposed to the right combination of growth factors, these CELLS may provide a unique source of replacement tissue for regenerative medicine. We have generated a mouse model to prove this concept by combining nuclear transfer with gene and cell therapy to treat a severe combined immunodeficiency disorder in mice.