Resistance to common chemotherapeutic drugs in ENKTL and CAEBV cell lines is not only due to P-glycoprotein–mediated drug efflux. Dose-response curves generated from annexin V/propidium iodide (PI) staining assays following 48-hour treatment with etoposide (A) or methotrexate (B) at the indicated concentrations. (C) The functionality of P-gp was assessed by the efflux of fluorescent rh-123 using flow cytometry. Cells were either left unstained or stained with rh-123 (black, filled histogram), stained with subsequent incubation at 4°C (gray, filled histogram), stained with subsequent incubation at 37°C (red, open histogram) or stained with subsequent incubation at 37°C in the presence of the P-gp inhibitor, CsA (blue, open histogram). (D) Rh-123 efflux was quantified using the following formula: (MFIred × 100) / (MFIgray − MFIblack). KG-1 cells (positive control) and Jurkat cells (negative control) were included in the experiments due to their reported high P-gp expression and lack of P-gp expression, respectively. Data are presented as means ± standard deviation (SD) of 3 independent experiments, each performed in triplicate (n = 3). ***P < .0001, vs Jurkat. CsA, cyclosporin A; MFI, median fluorescence intensity; P-gp, P-glycoprotein.