ABCG2 and breast cancer: In breast cancer cells, knockdown of β-catenin by siRNA resulted in increased chemosensitivity towards doxorubicin and etoposide (Bourguignon et al, 2009); although these MDR-associated drugs are also transported by other ABC transporters than ABCB1, ABCB1 is highly overexpressed in this cell line when compared with other MDR-related genes (Stein et al, 1997b).