Different forms of human -thalassemia are caused by mutations within in the -globin intron 2, which activate cryptic splice sites and thus lead to the formation of nonfunctional transcripts (Fig. (3B part I) ). Those aberrantly used sites can be blocked by antisense steric block oligonucleotides, which leads to the synthesis of functional protein [119, 120] . Kole and his group adopted this principle for the development of a splice correction assay [121] . In this model system, a firefly luciferase construct leads to the synthesis of inactive enzyme because the reporter gene pre-mRNA is interrupted by the human -globin intron 2 containing an aberrant splice site. Upon binding of a steric block oligonucleotide, correct splicing is restored which in turn yields a functional luciferase protein. To achieve this, the oligonucleotide has to be delivered to the nucleus. Furthermore, only oligonucleotides that don't activate RNase H are applicable [2] , e.g. phosphorodiamidate morpholino oligomers (PMO, [122] ), locked nucleic acids (LNA, [123] ), peptide nucleic acids (PNA, [124] ) or 2'-O-methyl-modified oligonucleotides (OMe). In addition to their inability to activate RNase H, most of these modifications confer higher affinity to the target RNA and increased resistance against enzymatic degradation than unmodified versions. Compared to RNAibased model systems, this assay is less susceptible to side effects like cytotoxicity or off-target effects because the reporter gene activity is turned up rather than turned down. The splice correction assay has been successfully applied for the analysis of several carrier systems [24, 93, [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] . In the following section, selected examples will be presented, which are summarized in Table 3 . In contrast to many noncovalent CPP-mediated siRNA delivery approaches, efficient splice correction was only achieved with conjugates of peptide and steric block oligonucleotide. Astriab-Fisher et al. [128] described delivery of OMe RNA phosphorothioate oligonucleotides linked via a disulfide bridge to Tat peptide and penetratin. A few hours after transfection, the CPP-oligonucleotide conjugates were detected both in cytoplasmic vesicles and in the nucleus and caused a dose-dependent increase in luciferase activity. These findings are in contrast to results of Turner et al. [93] , who could not find a biological effect for several CPP-oligonucleotide conjugates in a HeLa cell assay for Tat-mediated transactivation of the HIV-1 long terminal repeat. The authors observed vesicular uptake but no nuclear import for their highly pure conjugates. Interestingly, the rate of uptake could be enhanced by addition of free CPP to the conjugates, though still no biological activity was detected. Based on these findings, Turner et al. [93] concluded that these free CPPs form complexes with CPP-cargo conjugates, which play a significant role in the uptake process. This is in accordance with observations by Meade et al. [92] for the uptake of CPP-siRNA conjugates described above. Moulton et al. [136] achieved correction of missplicing at low micromolar concentrations of a R 9 F 2 -PMO conjugate but not with complexes of peptide and PMO. The steric block activity of the R 9 F 2 -PMO conjugates could be further increased with longer spacers whereas variations in the conjugation chemistry did not result in any differences. Furthermore, transfection rates were higher than for conjugates with Tat peptide, penetratin or a Tat peptide analogue. Using the HIV-1 transactivation assay mentioned above, Turner et al. [137] could show that most CPP-oligonucleotide conjugates attained biologic activity only through co-administration of the endosomolytic substance chloroquine. Fluorescence microscopy analyses revealed that this treatment released fluorescently labeled conjugates from endosomal compartments into the nucleus. Besides the addition of chloroquine, different endosome disrupting strategies have been evaluated using the splice correction assay, for example co-treatment with endosome-disruptive peptides [129] or photochemical internalization [138] (see chapter "Strategies to enhance endosomal escape"). However, the most promising results have been achieved with two newly developed derivatives of classical CPPs (reviewed in [130] ). The modification of oligoarginines with non-natural, uncharged amino acids [139] led, amongst others, to the peptide (R-Ahx-R) 4 , in which Ahx represents a six-atom aminohexanoic acid spacer. Abes et al. demonstrated that in contrast to Tat or oligoargine, PMO-conjugates of this peptide led to dose-dependent splice correction at low micromolar concentrations in the absence of endosomolytic agents. The underlying mechanism for this superior activity is not clear yet, as the uptake of (R-Ahx-R) 4 constructs was less efficient than the uptake of Tat or oligoarginine constructs and also involved endocytotic routes [126] . The second peptide is a derivative of penetratin, to which six arginine residues were added at the N-terminus (R 6 Pen). R 6 Pen-PNA conjugates were shown to promote efficient splice correction at low concentrations and in the absence of endosomolytic agents [127] . Again, uptake of R 6 Pen-conjugates seemed to involve endocytosis and there was hardly any difference in splice correcting activity regardless of the nature of the linker used for conjugation, e.g. a stable thioether versus a reducible disulfide linker [130] .