In addition, HEK293 cells transfected with either the ABCG2 or ABCB1 gene were also used, as unlike drug-selected cancer cells, which could become drug resistant due to numerous mechanisms or pathways, the gene-transfected cells should only be resistant to ABCG2 and ABCB1 substrates by overexpressing the ABCG2 or ABCB1 transporters. Here, ABCG2 is linked to cancer.