Bmi1 is also critical for the malignant transformation of stem cells.6, 7, 16 ALDH activity, which is a marker of CSCs, has been shown to be elevated in several tumor types, including brain, breast, liver, colon, pancreas and lung cancers.30 Overall, isolation of ALDH-positive cells from these tumors results in enrichment of tumor-initiating cells.31 In the present study, the ALDH assay revealed a significant increase in the number of ALDH-positive cells following forced Bmi1 expression in HPCs. Here, BMI1 is linked to lung cancer.