 A small population of tumor-initiating cells or cancer stem cells CSCs possess some biological functions like normal stem cells, including self-renewal, asymmetric cell division, slowly proliferation rate and drug-resistance. CSCs from many primary tumors and cell lines express specific stem cell markers, including Oct4, Sox2, Nanog, CD133 promimin-1, Nestin, CD44 ,CD24, ALDH Aldehyde dehydrogenase and c-Kit. There are many evidences that CSCs are responsible for tumor initiation, progression and metastasis. CSCs are also believed to have important roles in cancer recurrence due to their resistance to anti-cancer drugs and radiation. CSCs express high levels of ATP-binding cassette ABC transporters. ABC transporter can pump cytotoxic drugs out of cells and is one important mechanism of multidrug resistance in CSCs. In addition, CSCs have high reactive oxygen species ROS scavenger expression to remove ROS produced from irradiation therapy. 