Cationic lipoplex-or PEI-based transfection methods used to deliver charged AOs are not suitable for the delivery of uncharged AOs such as phosphorodiamidate morpholino oligomers (PMO, Figure 1 ) (1) and peptide nucleic acids (PNAs) (2) . Conjugation of PMO to short CPPs is a good method to enhance the cytoplasmic and nuclear delivery of PMO because the conjugates are simple to use and because the short peptides and their AO conjugates can be easily manufactured and characterized in a qualitycontrolled manner. Examples of well-studied CPPÀPMO conjugates include those with Tat and oligoarginine peptides (3, 4) Important considerations in the design of effective CPPs include the ability to deliver AO efficiently, stability in living systems and toxicity. We have reported that Tat and oligoarginine peptides are not stable in human serum (5) , and are therefore ill-suited for in vivo applications. Oligoarginine peptides incorporating non-a amino acids have been proven superior to oligoarginine alone. CPPs containing 6-aminohexanoic acid (X) and b-alanine (B) were more stable in human serum than Tat or oligoarginine peptides (5) . A CPPÀPMO conjugate, (RXR) 4 ÀPMO, has been shown to be more efficient in the correction of pre-mRNA mis-splicing (6) and in inhibition of the replication of mouse hepatitis virus in vivo (7) than an oligoarginine peptide. In addition, (RXR) 4 ÀPMO conjugates have been shown to cause effective exon skipping in muscle cells from DMD dogs (8) , in human muscle explants (9) and in mdx mice (10) , as well as inhibiting the replication of various viruses in cell cultures (7, (11) (12) (13) and in mice (7, 13) .