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Although all normal tissue cells,including stem cells,are genetically homologous,variation in gene expression pattes has already determined the distinct roles for individual cells in the physiological process due to the occurrence of epigenetic modification.This is of special importance for the existenee of tissue stem cells because they are exclusively immortal within the body,capable of selfreplicating and differentiating by which tissues renew and repair itself and the total tissue cell population maintains a steady-state.Impairment of tissue stem cells is usually accompanied by a reduction in cell number,slows down the repair process and causes hypofunction.For instance,chemotherapy usually leads to depression of bone marrow and hair loss.Cellular aging is closely associated with the continuous erosion of the telomere while activation of telomerase repairs and maintains telomeres,thus slowing the aging process and prolonging cell life.In normal adults,telomerase activation mainly presents in tissue stem cells and progenitor cells giving them unlimited growth potential.Despite the extensive demonstration of telomerase activation in malignancy(＞80%),scientists found that heterogeneity also exists among the tumor cells and only minorities of cells,designated as cancer stem cells,andergo processes analogous to the self-renewal and differentiation of normal stem ceils while the rest have limited lifespans.In this study,telomerase activity was measured and compared in breast cancer stem cells and non-stem cells that were phenotypically sorted by examining surface marker expression.The results indicated that cancer stem cells show a higher level of enzyme activity than non-stem cells.In addition,associated with the repair of cancer tissue(or relapse)after chemotherapy,telomerase activity in stem cells was markedly increased.