The effectiveness of each CPPÀPMO conjugate was determined in a previously described splicing correction assay (14) , considered a reliable method to assess nuclear antisense activity of a steric-blocking AO. This assay utilizes the ability of steric-blocking AOs to block a splice site created by a mutation in order to restore normal splicing. The luciferase coding sequence was interrupted by the human b-globin thalassemic intron 2 which carried a mutated splice site at nucleotide 705. HeLa cells were stably transfected with the plasmid therefore named as pLuc705 cell. In the pLuc705 system, steric-blocking AOs must be present in the cell nucleus for splicing correction to occur. Advantages of this system include the positive readout and high signal-to-noise ratio. With this system the relative efficiencies of various CPPs to deliver an AO with sequence appropriate for splice-correction to cell nuclei can be easily compared.