Modifications to the phosphate group in the sugar-phosphate backbone can improve the resistance of therapeutic RNAs to extracellular and intracellular nucleases. In addition, the negative charge on the phosphate group also interferes with the delivery of the RNA into the cell through the lipid bilayer membrane, which is impermeable to polar molecules. Thus, replacing the oxygens on the phosphates with neutral groups or complexing the phosphate groups to cations like sodium can improve RNA delivery. A widely used backbone modification, phosphorothioate as shown in Figure , which replaces an oxygen in the phosphate group with sulfur, reduces the activity of extracellular and intracellular nucleases. RNA molecules with phosphorothioate linkages at the ends resist exonucleases, whereas RNA molecules with phosphorothioate linkages within the RNA resist endonucleases. However, the sulfur on the phosphate group creates stereogenic α-phosphorus atoms resulting in diastereomers with different functional properties that can affect duplex formation. Careful spacing of the phosphorothioate linkages within the RNA can ameliorate this problem. Other backbone modifications replace the D-ribose with either an L-ribose or a non-ribose moiety (Figure ). Phosphorodiamidate morpholino oligonucleotides (PMO) contain morpholino groups linked by phosphorodiamidate groups rather than ribose linked by phosphates. Glycol nucleic acids (GNA) have a backbone of repeating glycerol units linked by phosphodiester bonds. In PNAs the sugar-phosphate backbone is replaced with a flexible N-(2-aminoethyl)glycine polymer with the nucleobases attached via a methylene carbonyl linkage. 263 PMO, GNA, and PNA all resist nuclease degradation. PNA forms duplexes with complementary DNA or RNA with higher affinity and specificity than unmodified DNA-DNA or DNA-RNA duplexes. However, duplexes containing PNA, PMO, and LNA resist RNase H degradation, inhibiting gene knockdown through targeted mRNA degradation. 259