This invention provides an oligonucleotide which has an improved affinity with Ago 2, wherein the oligonucleotide has a nucleotide residue or a nucleoside residue represented by formula (I) at the 5 end of the oligonucleotide, and wherein the nucleotide residue or the nucleoside residue binds to an adjacent nucleotide residue via the oxygen atom at 3-position: [wherein X1 represents an oxygen atom or the like R1 represents formula (IIA), (IVA), or the like (wherein R5A represents halogen or the like, and R6A represents hydrogen atom or the like) (wherein Y3A represents nitrogen atom or the like, and Y4A represents CH or the like) R2 represents hydrogen atom, hydroxy, halogen, or optionally substituted lower alkoxy R3 represents hydrogen atom, or (wherein n2A represents 1, 2, or 3)].本發明提供一種與AGO2之親和性提高之寡核苷酸等,其於寡核苷酸之5末端具有式(I)所表示之核苷酸殘基或核苷殘基,且該核苷酸殘基或核苷殘基經由3位之氧原子而與鄰接之核苷酸殘基鍵結,{式中,X1為氧原子等,R1為下述之式(IIA)、或式(IVA)等,(式中,R5A為鹵基等,R6A為氫原子等)(式中,Y3A為氮原子等,Y4A為CH等),R2為氫原子、羥基、鹵基或可具有取代基之低級烷氧基,R3為氫原子、或(式中,n2A為1、2或3)}。
