Until recently, various tissue culture studies have consistently shown that MDR in most cultured cancer cells involves ATP-binding cassette (ABC) transporters in the human such as P-glycoprotein (P-gp, ABCB1) [19–21], multidrug resistance protein (MRP, MRP-1/ABCC1) [22–24], breast cancer resistant protein (BCRP, ABCG2) [25], lung resistant protein (LRP) [26–28], bcl-2 [29], p53 [30,31], Topoisomerase II (TopoII) [32,33], etc. Here, ABCG2 is linked to cancer.