In vitro assays: exon skipping in mdx mouse myotubes. H2K mdx myotubes were prepared and incubated with peptide-PMO conjugates in the absence of any transfection agent at concentrations of 0.125, 0.25, 0.5, and 1.0 µmol/l by the method described previously. 29 The products of nested RT-PCR from total isolated RNA were examined by electrophoresis on a 2% agarose gel. Quantification of ∆23 transcript levels was calculated using densitometry. The MTS cell viability test (Promega, Madison, WI) showed 100% survival at the highest concentrations of peptide-PMO conjugates used in the study (data not shown).