CPP nomenclature and sequences are listed in Table 1 . Chemical structures of PMO and (RX) 8 ÀPMO are shown in Figure 1 . The antisense PMO (CCT CTT ACC TCA GTT ACA) is designed to target a b-thalassemic mutant splice site present in the human b-globin intron 2 of a positive-readout antisense activity assay system (13) as described in the Results section. Synthesis of PMO, described previously (15, 16) , and the CPPs, using standard Fmoc chemistry (17) , were performed at AVI BioPharma, achieving purities of 490% as determined by HPLC and mass spectrometry analysis. Conjugation of a CPP to a PMO through an amide linker, described previously (6) , was followed with an additional purification step to remove nonconjugated peptide. Samples were loaded on source 30S resin (Amersham Biosciences, Pittsburgh, PA) in a 2 ml Biorad (Hercules, CA) MT2 column at 2 ml/min with running buffer A (20 mM Na 2 HPO 4 , 25% acetonitrile, pH 7.0) and purified into 45-s fractions with 0-35% buffer gradient (buffer B: 1.5M NaCl, 20 mM Na 2 HPO 4 , 25% acetonitrile, pH 7.0) over 60 min, using a Biorad BioLogic low pressure chromatography system. The desired faction was desalted by a method described previously (6) . HPLC and MS analyses revealed that the final product contained 490% CPP conjugated to fulllength PMO, with the balance composed of CPP conjugated to incomplete PMO sequence, nonconjugated full-length or incomplete PMO.