Given that doxorubicin is also a substrate of ABCG2 [60], the inhibition of ABCG2 and the resultant decreased efflux of doxorubicin likely underlie the observed synergism, similar to the interaction seen with the EGFR inhibitor gefitinib and doxorubicin in thyroid cancer [61]. This evidence concerns the gene ABCG2 and thyroid cancer.