As already mentioned, (R-Ahx-R) 4 -PMO conjugates become cytotoxic in murine models at high doses. The cytotoxicity of nonviral delivery vectors for nucleic acids is generally associated to their resulting cationic charge. We therefore investigated whether reducing the number n of repeats in (R-X-R) n -PMO conjugates could be possible. A significant loss of splicing correction efficiency was found with shorter versions of these PMO conjugates as shown for (R-AbuL-R) n -PMO conjugates ( Figure 5 ).