Two models have been suggested to explain the connection between MDR and CSCs: the “original” MDR model according to which, after exposure to the chemotherapeutic agent, only CSCs expressing ABC transporters repopulate the tumor, and the “acquired” MDR model according to which after chemotherapy, only CSCs survive and this population of survival cells, after mutations, originates new and more aggressive drug-resistant cell phenotypes. This evidence concerns the gene ABCG2 and neoplasm.