The present invention is directed to RNA oligonucleotides or complexes of RNA oligonucleotides, denoted herein together as RNA complexes, containing at least one, but optionally more that one, hydroxymethyl substituted nucleotide monomer(s). By a hydroxymethyl substituted nucleotide monomer is understood a nucleotide monomer containing a hydroxymethyl group (that may be unsubstituted, O-substituted for example with a conjugating group, or converted into an optionally substituted or conjugated aminomethyl group). This hydroxymethyl group is not partaking in formation of an internucleotide linkage and is not the hydroxymethyl group (containing the 5"-hydroxy group) of a natural RNA monomer. The RNA complexes of the invention may be useful for therapeutic applications, diagnostic applications or research applications. The complexes include short interfering RNA complexes (siRNA duplexes) comprising an antisense strand and a continued or a discontinued passenger strand ("sense strand") capable of regulating gene expression. At least one of these strands, possible more than one of these strands, are modified with one or more hydroxymethyl substituted nucleotide monomer(s) of this invention, that can be positioned at the 3"-end, at the 5"-end or internally. The RNA complexes of the invention can also be single stranded RNA oligonucleotides ("RNA strands") that are modified with at least one, but optionally more that one, hydroxymethyl substituted nucleotide monomer(s). Such single stranded RNA strands are to be considered included whenever the term RNA complexes is used in this patent application. The complexes of the invention display enhanced stability towards nucleolytic degradation relative to the corresponding complexes comprised entirely from natural RNA monomers.
