*2.3.3.2 Phosphoramidate morpholino oligomer*

PMOs are non-charged ASOs whose pentose sugar is substituted by a morpholino ring and the inter-nucleotide linkages are phosphoramidate bonds in place of phosphodiester bonds [41] (**Figure 2d**). PMOs avoid the RNase H recruitment; their effect is primarily mediated by steric interference of ribosomal assembly resulting in translational arrest. Synthetic PMOs are resistant to nucleases degradation in biological fluid. Because their backbone is uncharged, PMOs are unlikely to form unwanted interactions with nucleic acid-binding proteins [42]. PMOs do not enter mammalian cells easily in culture, but it has been shown that conjugating it with peptides such as arginine-rich peptide (ARP) can enhance its cellular uptake and antisense potency [43]. Antisense PMO oligonucleotide, have shown efficacy in animal models *in vivo* and in human clinical trials [44, 45]. Indeed, a PMO antisense agent is currently in phase II clinical trials for restenosis, cancer and polycystic kidney disease.
