CD44 and lung cancer: Cancer stem cells are a small subpopulation of tumour cells that possess similar characteristics to normal stem cells.5,6 Cancer cells with high stemness have high proliferative, metastatic, and drug-resistance capabilities.7 Different cancer stem cells express different CD antigens.7 CD antigens used as markers of lung cancer stem cell include CD44, CD90, CD117, CD133, and CD166.8 Different from other CD antigens of lung cancer stem cell markers, the complete function of CD133 remains unclear to date.