The naturally occurring CPPs such as Tat peptide are not stable in blood and neither are oligolysine/oligoarginine (5) , rendering these CPPs unfavorable as transporters for therapeutic AOs. We reasoned that one approach to improve stability would be to use non-a amino acids or D-amino acids. In this study, we investigated whether incorporation of 6-aminohexanoic acid (X), b-alanine (B) and D-arginine (r) amino acids into the CPP would affect cellular delivery, antisense activity, toxicity and serum binding of the resulting CPPÀPMO conjugates.