Synthesis of modified nucleotides and then incorporation into an RNA chain have facilitated our understanding of the functron and mode of action of specific nucleotides within the ribonucleic acid. Recent developments in the chemical and enzymatic synthesis of RNAs (1 –3 ) have allowed the introduction of single-atom substitutions at specific positions in an oligoribonucleotide. For example, introduction of deoxyribose moieties in an RNA or the replacement of natural nucleotide by a base not normally found in nucleic acids is now possible (4 ,5 ).
