Cancer stem cells (CSCs) develop mechanisms of drug resistance like a high expression of membrane-bound efflux pumps, especially ATP-binding cassette (ABC) transporters such as ABCB1 (also known as MDR1 or P-glycoprotein) and detoxifying enzymes like the aldehyde dehydrogenase ALDH1A1, a cytosolic enzyme that affords protection from the adverse effects of chemotherapeutic insult. This evidence concerns the gene ALDH1A1 and cancer.