Immunostaining CHO or CHO/XPR1 cells were fixed using 4% paraformaldehyde in PBS, permeabilized using 0.5% Triton X-100 and stained using anti-XPR1 and anti-rabbit IgG coupled to FITC. Before mounting the slides, cells were counterstained with the nuclear stain DAPI. Pictures were taken using a fluorescence microscope (Carl Zeiss, Goettingen, Germany) and images were processed using the ImageJ software (U.S., National Institutes of Health).