ABCG2 and cancer: In this study, the three natural compounds 6-methoxymellein (3), angelicoin B (4) and ellagic acid as well as nine novel 3,4-dihydroisocoumarins (Figure 1) were analyzed regarding their cytotoxicity in cancer cells and inhibition of the endogenously expressed human ABC transporters P-gp, BCRP, and MRP1 and of the yeast transporter Pdr5.