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Oligonucleotide modifications. a The chemical structure of unmodified RNA bases. The 2′ OH group in green is the distinguishing feature and target of modifications. b Phosphorothioate modification, where the non-bridging oxygen on the phosphate linkage is replaced with a sulfur atom (red); c additional 2′-O- substitutions; d locked nucleic acids, where the ribose moiety is modified with an extra bridge connecting the 2′ oxygen and 4′ carbon
