In early studies [38,39], it was found that 3,3′,4′,7-tetra-O-methylated quercetin 19 and 3,3′,4′,5,7-penta-O-methylated quercetin 20 could represent potential anti-multidrug resistance (MDR) agents due to their ability to influence the effects of breast cancer resistance protein (BCRP), which is known to determine resistance in cancer cells. Here, ABCG2 is linked to cancer.