ABCG2 and cancer: A family of ABC transporters have been characterized to act specific physiological functions such as ABCA1 for cholesterol efflux onto high-density lipoproteins, while only a few of ABC transporters are identified to be responsible for multidrug resistance by efflux of drug from cancer cells, such as ABCB1, ABCC1 and ABCG2, etc. [24].