We have recently shown that acquired resistance to SN-38 in human CRC and breast cancer cell lines results in a major upregulation of the ABCG2 transcript and protein, and that the specific inhibition of ABCG2 results in the re-sensitization of the resistant cells [9,10], which strongly suggests a key role of this protein in SN-38 resistance. Here, ABCG2 is linked to breast carcinoma.