Fas expression and internalization of QDs. a. Exposure to QDs induces cell-surface Fas expression in neuroblastomas. Fas expression was assessed by FACS in untreated cells (grey line) and in cells exposed to cysteamine-QDs for 24 hours (black line). Dotted line shows background staining of untreated cells with isotype-matched control antibody. b. Net Fas expression was calculated as Mean Fluorescence Intensity (MFI) of cells stained with anti-Fas antibodies subtracted by MFI of isotype control antibody-stained cells. Averages and standard deviations from three independent experiments (N = 9) are shown (*p < 0.05). c. QD uptake was assessed by flow cytometry in neuroblastoma cells treated with 5 μg/ml QDs (unmodified and NAC-modified) for 24 hours. d. Net Mean Fluorescence Intensities (MFI) of cells treated with cysteamine-QDs, NAC-conjugated and NAC-capped QDs, and cysteamine-QDs in the presence of 2 mM NAC (NAC + QD) are shown. Net MFI was calculated as MFI of QD-treated cells subtracted by the autofluorescence of untreated cells. Note a significant decrease (p < 0.05) in the MFI of NAC-modified QDs compared with MFI of cysteamine-QDs. Averages and standard deviations from three independent experiments (N = 9) are shown (***p < 0.001).