Fig. 5. Maintenance of normal tissue homeostasis and a hypothetical model of the cancer-stem-cell-based origin of tumors. (A) Asymmetric cell divisions of stem and progenitor cells result in the generation of differentiated non-proliferating cells, which often constitute the bulk of the normal tissue. (B) Abnormal asymmetric cell division of stem and progenitor cells might be responsible for the accumulation of cells that fail to withdraw from the cell cycle and continue to divide. In this scenario, both progenitor- and stem-cell populations are collectively known as cancer stem cells.