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population whose primary function is to generate progeny that then develop into terminally differentiated cell types, such as a cardiomyocytes, neurons or photoreceptors. Tissuespecific adult stem cells, or progenitors, are committed to producing tissue or lineage-specific cells, whereas totipotent or pluripotent stem cells can give rise to any of the 200+ cell types of the body. There are two types of pluripotent stem cells defined by their tissue origin: 1) embryonic stem (ES) cells obtained from early embryos, typically at the blastula stage, and 2) induced pluripotent stem (iPS) cells derived through a reprogramming process whereby terminally differentiated somatic cells are reprogrammed or induced to a pluripotent state (Figure 1). Successful generation of human iPS cells was reported independently in 2007 by the research groups of Drs James Thomson and Shinya Yamanaka1,2. Both groups successfully identified a minimum number of nuclear factors that could reprogramme terminally differentiated fibroblasts to pluripotent cell lines by exogenously expressing distinct yet overlapping sets of genes: Yu and colleagues used Oct4, Sox2, Nanog and Lin28 by lentiviral gene transfer, while Yamanaka and colleagues employed Oct4, Sox2, Klf4 and c-Myc via retroviral gene transfer. These landmark studies were a significant advance over whole nucleus reprogramming3 and have had several important consequences. Induced pluripotent stem cell technology does not require human embryos and thus circumvents the ethical issues associated with By Dr Dwight Morrow and Dr Julie Holder