Our previous lead Pip series CPP, Pip5e, 29 contains two arginine-rich flanking regions and a central hydrophobic core. To further probe the composition requirements of the hydrophobic core for maintenance of good heart dystrophin production, we synthesized a range of Pip5e derivative peptides (Pip6 a-f) (Figure 1a) where mutations were made only to the hydrophobic core region, for example scrambled and partially deleted core region peptides. All peptides contained the same number of arginine residues (10) in the flanking sequences as in Pip5e, with the exception of Pip6e. These peptides were conjugated to a 25-mer PMO complementary to dystrophin exon 23, 30,31 previously validated for exon skipping in mdx mice. In contrast to the method of conjugation to the 5′ end of PMO that we utilised previously, 29 Pip6-PMO conjugates were prepared by conjugation of the 3′ end of the PMO to the C-terminal carboxylic acid moiety of the Pip peptide (Figure 1b) . We reported that there was no significant difference between the in vivo dystrophin production or exon skipping activity for Pip5e-PMO conjugated to the 3′ end of the PMO or to the 5′ end and therefore chose to utilise 3′ end conjugation for these experiments. 32