Riboses 2 hydroxy group reacts with the phosphate linked 3 hydroxy group, making RNA too unstable to be used or synthesized reliably. To overcome this, a ribose analogue can be used. The most common RNA analogues are 2-O-methyl-substituted RNA, locked nucleic acid (LNA) or bridged nucleic acid (BNA), morpholino, and peptide nucleic acid (PNA). Although these oligonucleotides have a different backbone sugar—or, in the case of PNA, an amino acid residue in place of the ribose phosphate—they still bind to RNA or DNA according to Watson and Crick pairing while being immune to nuclease activity. They cannot be synthesized enzymatically and can only be obtained synthetically using the phosphoramidite strategy or, for PNA, other methods of peptide synthesis.