Surprisingly, these changes to the hydrophobic core resulted in further reductions in dystrophin expression not only in heart but also in all other tissues, as observed in the immunohistochemical staining (Figure 7a ) and in the quantifications thereof (Figure 7b,c) . The immunohistochemical staining representative images revealed very few dystrophin positive fibers in all tissues with the exception of the TA and quadriceps (Figure 7a) . With reference to the quantifications, Pip6g-PMO was not significantly different to untreated mdx in the quadriceps or diaphragm (Supplementary Figure S3a) . Statistical significance tables for immunohistochemical staining of quadriceps, diaphragm, and heart muscles for Pip6a-f-treated mdx mice relative to untreated mdx mice. Statistical significance was determined using a repeated measures, multilevel statistical model (****P < 0.0001, ***P < 0.001, **P < 0.01*P <0.05, N/S, not significant). Statistical significance tables for immunohistochemical staining of quadriceps, diaphragm, and heart muscles for Pip6a-f-treated mdx mice relative to Pip5e-treated mice. Statistical significance was determined using a repeated measures, multilevel statistical model (****P < 0.0001, ***P < 0.001, **P < 0.01*P <0.05, N/S, not significant). Both Pip6e-PMO derivatives were not significantly different to untreated mdx in heart muscles, illustrating the general inefficiency of these two peptides. Similarly, these Pip6e-PMO derivatives showed reduced efficiency in exon skipping, as illustrated in representative RT-PCR images (Figure 7e ) and in qRT-PCR analyses (Figure 7d and Supplementary Figure S3c ) in all tissues. Western blots revealed negligible dystrophin protein restoration (Figure 7f and Supplementary Figure S3d ) in all tissues with the exception of the TA muscle. These data show that an increase in the number of arginines or alteration in the location of the single arginine in the hydrophobic region of Pip6e are detrimental to both heart as well as skeletal muscle dystrophin production.