The establishment of human ES cells has given tremendous input to the concept of stem cell-based therapies and the parallel development of somatic nuclear transfer has opened the perspective of autologous stem cell therapy in human medicine. Somatic cell nuclear transfer is currently the only procedure that allows the complete reprogramming of the genome of a differentiated cell into an embryonic stem cells (nuclear transfer stem cells, NTSC). Enucleated matured oocytes are commonly used, however other stages of oocyte maturation or oocytes from a species different from the nuclear donor offer opportunities to overcome biological and / or practical limitations. Nuclear donor pre-treatment with drugs acting on chromatin structure or exposure to cell extract may provide an ex ovo reprogramming strategy. Understanding the events taking place during this nuclear-cytoplasmic interaction will provide the basis for genome reprogramming in vitro of somatic cells and therefore the creation of immunological compatible cells for cell replacement therapies, combining the potential of embryonic stem cell technology to the theoretical generation of any given somatic tissue. However, evidence for epigenetic defects in cloned embryos and lack of cell transplantation models are raising concerns on the long-term safety of such applications. For all of these reasons, there is an urgent need to develop strategies to improve the fidelity of reprogramming and to investigate new routes for stem cell derivation directly from somatic cells including interspecies nuclear transfer and in vitro reprogramming (ex-ovo reprogramming). Here we propose a comparative and multidisciplinary approach that uses non human mammals (cattle, sheep, pig and mouse), as model systems for the derivation, molecular and epigenetic characterization and in vitro/in vivo differentiation of NTSC lines. The project is expected to shed light on the basic features of nuclear transfer embryonic stem cells across species responding to the need for new animal models and encouraging comparative approaches.