Given that the direct inhibition of the efflux function of the drug transporter remains the most effective strategy for overcoming transporter-mediated MDR in cancer cells [18,58,59,60], we examined the effect of imperatorin on ABCG2-mediated efflux of an ABCG2-specific fluorescent substrate PhA [61] in S1-MI-80 and H460-MX20 cancer cells. Here, ABCG2 is linked to cancer.