Furthermore, ABC transporters such as multidrug-resistance-associated protein 1 (MRP1), multidrug-resistance protein 1 (MDR1), and ATP-binding cassette super-family G member 2 (ABCG2) are overexpressed in the CSCs of breast cancer, pumping out chemicals such as anti-cancer drugs or Hoechst 33342 [18]. Here, ABCB1 is linked to breast cancer.