SP cells are identified as a drug resistant fraction by their ability to efflux a Hoechst 33342 dye, a substrate for the ATP-binding cassette (ABC) transporter ABCG2 [44], and have been reported to contain CSCs in many cancers._ENREF_45 We found that HM1.24 was highly expressed on the surface of SP cells, and that YB-AHM plus Len in combination effectively reduced SP fractions in RPMI 8226 and KMS-11 cells (Figure 4B). Here, ABCG2 is linked to cancer.