It is an object of the present invention to provide a novel molecule for antisense therapies which is not susceptible to nuclease degradation in vivo and has a high binding affinity and specificity for the target mRNAs and which can efficiently regulate expression of specific genes. The novel artificial nucleosides of the present invention has an amide bond introduced into a bridge structure of 2, 4 -BNA/LNA. The oligonucleotide containing the 2, 4 -bridged artificial nucleotide has a binding affinity for a single-stranded RNA comparable to known 2, 4 -BNA/LNA and has an increased nuclease resistance over LNA. Particularly, it is expected to be applied to nucleic acid drugs because of its much stronger binding affinity for single-stranded RNAs than S-oligos affinity
