Pluripotential stem cells have been derived in mice and primates from preimplantation embryos, postimplantation embryos and bone marrow stroma. Embryonic stem cells established from the inner cell mass of the mouse and human blastocyst can be maintained in an undifferentiated state for a long time by continuous passage on embryonic fibroblasts or in the presence of specific inhibitors of differentiation. Pluripotential stem cells can be induced to differentiate into all the tissues of the body and are able to colonise tissues of interest after transplantation. In mouse models of disease, there are numerous examples of improved tissue function and correction of pathological phenotype. Embryonic stem cells can be derived by nuclear transfer to establish genome-specific cell lines and, in mice, it has been shown that embryonic stem cells are more successfully reprogrammed for development by nuclear transfer than somatic cells. Pluripotential stem cells are a very valuable research resource for the analysis of differentiation pathways, functional genomics, tissue engineering and drug screening. Clinical applications may include both cell therapy and gene therapy for a wide range of tissue injury and degeneration. There is considerable interest in the development of pluripotential stem cell lines in many mammalian species for similar research interests and applications.