To analyze (R-X-R) 4 -PMO conjugates intracellular distribution, exponentially growing HeLa pLuc 705 cells (3.5 Â 10 4 cells seeded and grown overnight in 2 ml culture dishes) were washed with OptiMEM and incubated with 2 mM FAM-labeled (R-X-R) 4 -PMO in the absence or in the presence of 20 mg/ml saponin for 30 min in OptiMEM medium. Cells were then washed with PBS prior a co-incubation step with 10 mg/ml Transferrin-Alexa 546 (red fluorescence) and Hoechst 33342 dye (blue fluorescence) for 10 min in order to stain endosomes and nuclei, respectively. The distribution of fluorescence in live unfixed cells was analyzed on Zeiss Axiovert 200M fluorescence microscope (Carl Zeiss, Obercochen, Germany).