Besides, BCRP, other known HIF-1 target genes that may serve as markers of letrozole resistance include: 1) cancer stem cell maintenance markers (Oct-4, kit ligand, JARID1B); 2) epithelial-mesenchymal-transition (EMT) markers (Snail, vimentin); and 3) invasion markers (c-Met, endothelin 1, fibronectin, MMP-2 and -4)[27,69]. Here, MMP2 is linked to cancer.