a phosphoramidate type bond wherein one of two non crosslinked oxygen atoms in the phosphodiester bond is replaced with an amino group or a substituted amino group a phosphorothioate type bond wherein one of two non crosslinked oxygen atoms in the phosphodiester bond is replaced with a sulfur atom or a phosphorodithioate type bond wherein each of two non crosslinked oxygen atoms in the phosphodiester bond is replaced with a sulfur atom the oligonucleotide of the present invention may contain one or more modified phosphodiester bonds as above in one or more internucleotide bonds the modified phosphodiester bond is preferable as the internucleotide bond from the standpoints of the specificity in molecular recognition a stability of the double stranded chain a resistance to a nuclease a penetrating property through a cell membrane a low cytotoxicity a moderate metabolizability an easy procedure for preparation and so on further the phosphorothioate type bond is more preferable from the standpoint of a stability in a living body it is particularly preferable that not less than half or in particular all of the internucleotide bonds are the modified phosphodiester bonds in particular the phosphorothioate type bonds the oligonucleotide of the present invention may be prepared in accordance with methods which are in themselves known for example the hairpin type oligonucleotide or dumbbell type oligonucleotide of the present invention.