ABCG2 and lung cancer: SP cells have high homology, self-renewal and multidirectional differentiation potential.47,48 Some reports have shown that ABCG2 is highly expressed in SP cells.47,49 ABCG2 is highly related to the drug resistance of CSCs and is used as a phenotypic marker for CSCs,50,51 including ovarian cancer,52 AML,53 breast cancer,54 lung cancer,55 nasopharyngeal carcinoma,56 and hepatocellular carcinoma (HCC).57 Montanaro et al.58 explored the optimal concentration of Hoechst 33342 to reduce the toxic effect.