The present invention relates to pyrazolotriazolyl nucleoside analogues, oligonucleotide comprising them, and uses thereof. Further the invention relates to a method for reducing gene expression in a cell comprising transfecting the cell with such an oligonucleotide. It has been found, in accordance with the present invention, that certain pyrazolotriazolyl-based nucleoside analogues such as 8-[3&prime-(2-cyanoethyl)-(N,Ndiisopropyl)]-phosphoramidite-(2&prime-deoxy-5&prime-dimethoxytrity 1-13-D-ribofuranozyl)-4-{N-benzoylamino)-pyrazolo[1,5a]-1,3,5-triazine, can be incorporated into oligonucleotides, substituting natural purine nucleosides and imparting acid stability and nuclease stability to the oligonucleotide. Furthermore, oligonucleotides comprising one or more such nucleoside analogues are capable of hybridizing with oligonucleotides having complementary sequences thereto in which no such analogues are incorporated.
