An oligonucleotide having a nucleotide residue or a nucleoside residue represented by formula (I) {wherein X1 is an oxygen atom or the like, R1 is formula (IIA) (wherein R5A is halogen or the like, and R6A is a hydrogen atom or the like), formula (IVA) (wherein Y3A is a nitrogen atom or the like, and Y4A is CH or the like), or the like, R2 is a hydrogen atom, hydroxy, halogen, or optionally substituted lower alkoxy, and R3 is a hydrogen atom or the like, or formula (VI) (wherein n2 is 1, 2 or 3)} at the 5′ end thereof, wherein the nucleotide residue or the nucleoside residue binds to an adjacent nucleotide residue through the oxygen atom at position 3, is provided.
