Novel oligonucleotide pairs which can form a duplex comprising one or more DNA-like nucleotides (e.g.,2'-sub-stituted arabinonucleotides (ANA)); in combination with one or more RNA-like nucleotides (e.g., 2'-substituted ribonucleotides(RNA) and/or locked nucleic acid nucleotides (LNA)), are disclosed. The use ofsuch oligonucleotide duplexes, such as forsilenc-ing the expression of a nucleic acid or gene of interest using smallinterfering RNA (siRNA) technologies, is also disclosed.