Fig. 4

Chemical modifications used for small nucleic acids. Common modifications can be divided into three categories: ribosome modifications, nucleobase group modifications and backbone modifications. Modifications of ribosomes are always performed on the 2’ ends of ribose sugars (blue box) and sugar chains (orange box). In modifications of the base group, uracil, adenine and cytosine are the common objects (purple box), and the ribose sugar can be completely replaced (red box). The PO linkage of the backbone can be changed, including the nonbridging O atom (cyan box), 3’-bridging O atom (green box) and total PO linkage (golden box), to reduce the net anionic charge of small nucleic acids