We now report the results of a series of mutations to the hydrophobic core region of the Pip5e peptide, where this central core region amino acid sequence is reversed, scrambled, or partially deleted. These changes affect the levels of exon skipping and dystrophin restoration in multiple muscle groups, including the heart, following a single, low dose intravenous injection of the corresponding Pip6-PMO conjugates. The results show that a core length of 5 amino acids (5-aa) appears to be essential for heart dystrophin production, since reductions in core length reduced cardiac activity. Unexpectedly, an arginine residue was tolerated in one position of the hydrophobic core, but two arginine residues were not tolerated, nor an arginine in a different position. Surprisingly, skeletal dystrophin production was also reduced in these two latter cases.