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circumvented only by continuous administration of immunosuppressive drugs, and the drugs themselves may cause deleterious side effects.VI. What are induced pluripotent stem cells?Induced pluripotent stem cells (iPSCs) are adult cells that have been genetically reprogrammed to an embryonic stem cell–like state by being forced to express genes and factors important for maintaining the defining properties of embryonic stem cells. Although these cells meet the defining criteria for pluripotent stem cells, it is not known if iPSCs and embryonic stem cells differ in clinically significant ways. Mouse iPSCs were first reported in 2006, and human iPSCs were first reported in late 2007. Mouse iPSCs demonstrate important characteristics of pluripotent stem cells, including expressing stem cell markers, forming tumors containing cells from all three germ layers, and being able to contribute to many different tissues when injected into mouse embryos at a very early stage in development. Human iPSCs also express stem cell markers and are capable of generating cells characteristic of all three germ layers.Although additional research is needed, iPSCs are already useful tools for drug development and modeling of diseases, and scientists hope to use them in transplantation medicine. Viruses are currently used to introduce the reprogramming factors into adult cells, and this process must be carefully controlled and tested before the technique can lead to useful treatments for humans. In animal studies, the virus used to introduce the stem cell factors sometimes causes cancers. Researchers are currently investigating non-
14viral delivery strategies. In any case, this breakthrough discovery has created a powerful new way to “de-differentiate” cells whose developmental fates had been previously assumed to be determined. In addition, tissues derived from iPSCs will be a nearly identical match to the cell donor and thus probably avoid rejection by the immune system. The iPSC strategy creates pluripotent stem cells that, together with studies of other types of pluripotent stem cells, will help researchers learn how to reprogram cells to repair damaged tissues in the human body.VII. What are the potential uses of human stem cells and the obstacles that must be overcome before these potential uses will be realized?There are many ways in which human stem cells can be used in research and the clinic. Studies of human embryonic stem cellswill yield information about the complex events that occur during human development. A primary goal of this work is to identify how undifferentiatedstem cells become the differentiated cells that form the tissues and organs. Scientists know that turning geneson and off is central to this process. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell divisionand differentiation. A more complete understanding of the genetic and molecular controls of these processes may yield information about how such diseases