Immunohistochemical staining of dystrophin expression for all 5-aa core Pip6-PMOs revealed high levels of dystrophin production in skeletal muscles including the TA, diaphragm, and the heart (Figure 3 ). Immunohistochemical staining quantification (Figure 4a , b) was performed as previously described 16, 34 and was achieved by taking four representative frames of the dystrophin staining and correlating this with laminin staining for each section (n = 3) of the quadriceps, diaphragm and heart for each peptide-PMO treatment. Untreated mdx and treated mdx mice were normalized to C57BL10 mice. This method allows comparison of the staining intensity of dystrophin at the sarcolemma relative to laminin for each treatment group. Intensity ratios are normalized to C57BL10 samples and each region of interest at the sarcolemma (120 regions for each treatment group) is plotted on a scatter graph. The relative intensity values obtained for all four of the 5-aa core Pip6-PMO conjugates were significantly different to those of untreated mdx mice for the quadriceps and diaphragm ( Figure 4b and Table 1 ). There were very similar dystrophin restoration levels in the quadriceps (percent recovery score-%RS-range between 21.10 and 33.44%; Figure 4b ) and in the diaphragm for all treatments, with the exception of Pip6b which had a higher recovery score in the diaphragm (%RS range between 38.87 and 48.43%, Pip6b 56.72%; Figure 4b ).