Our data have confirmed that cellular uptake is not the limiting factor in the efficiency of splicing correction by (R-X-R) 4 -PMO conjugates and therefore intracellular trafficking and endosomal escape likely to be major limiting factors. To evaluate the ability of CPP-PMO conjugates to escape from endosomes we employed a liposome leakage assay. Late endosomes are characterized by a rather unusual lipid composition enriched in LBPA (23) and have a pH 5.5 lumen (24) . We therefore prepared liposomes from a lipid mixture mimicking the lipid composition of late endosomes DOPC/DOPE/PI/LBPA (5:2:1:2) and monitored the effect of low pH on the CPP-PMO induced leakage of a fluorescent dye entrapped in the lipid vesicles. We compared conjugates 1, 5, 9 and (Arg) 8 -PMO. All conjugates induced a fairly modest leakage that was strongly promoted at pH 5.5. In correlation with the data on splicing activity, (R-Ahx-R) 4 -PMO conjugate was by far the most active in this group, followed by (R-G-R) 4 endosomal escape is a major contributing factor for the efficient nuclear delivery of these CPP-PMO conjugates.