PURPOSE: To provide a modified oligonucleotide that includes at least one of 8-azapurine base, forms a more stable hybridization complex with a nucleic acid, can inhibit gene expression, detection of nucleic acids and is useful as a pharmaceutical agent and as a diagnostic agent.

CONSTITUTION: This new modified oligonucleotide that is useful as a pharmaceutical agent or a diagnostic agent is represented by formula I [R1 is H, a 1-18C alkyl, a 2-18 alkenyl, a 2-18C alkylcarbonyl, a group of formula II (Z and Z' are each H, a 1-18C alkyl, a 2-18 alkenyl, a group of formula III (X is OH, SH)); V is oxy, sulfanediyl, imino; Y is oxy, sulfanediyl, imino, methylene; Y' is oxy, sulfanediyl, imino, alkylene; V is OH, SH, a 1-18C alkoxy, a 1-18C alkyl, a 6-20C aryl; a is O, CH2; R2 is H, OH, a 1-18C alkoxy, NH2; n is 3-99; B is a base, one of B is a group of formula IV (E and F are each H, OH, NH2)].