ABCG2 and cancer: The major ABC transporters responsible for resistance to anti-cancer chemotherapeutics include MDR-1 (P-glycoprotein) encoded by the human ABCB1 gene located on chromosome 7q21 [4, 5, 7] and Breast Cancer Resistance Protein (BCRP) encoded by the human ABCG2 gene located on chromosome 4q22 [1, 3, 8–10].