ABCG2 and Epstein-Barr virus infection: In order to explore the biological characteristics and mechanism of drug-resistance of these cells, SNK-6, SNK-6/ADM and SNK-6/ADM-SP cells were utilized to evaluate potentially differences of chemotherapy resistance index (RI), morphology, proliferation, cell cycles, expression of ATP-binding cassette (ABC) transporters (ABCG1, ABCG2 and ABCC4) and surface markers, cytokine sensitivity, and situation of EBV infection.