Monomeric units of nucleic acids DN A or RN A are characterized by a β configuration at the anomeric center of the sugar moiety (Fig. 1 ). Natural constitutive nucleosides of this configuration are readily commercially available, and from such starting building blocks, numerous phosphate-backbone-modified oligonucleotides have been proposed, the most well known being the methylphosphonate and the phosphorothioate series ( 1 ). ? Fig. 1.? Monomeric structure of oligonucleotides.