Oligonucleotides labeled with stable isotopes and a method for detecting the same

An antisense oligonucleotide sequence or antisense peptide nucleic acid sequence comprising a single strand RNA or single strand or double strand DNA containing at least one nucleotide or peptide nucleic acid as a structural unit, in which at least one structural C atom is substituted by 13C, and at least one structural N atom is substituted by 15N, having 10 to 100 bases complementary to the desired target sequence to be hybridized,

• said oligonucleotide being
• 1) a natural oligonucleotide wherein 3′-OH and 5′-OH in ribose or deoxyribose are cross-linked by phosphodiester bonds,
• 2) a phosphorothioate oligonucleotide wherein one or two non cross-linked oxygen atoms in the phosphodiester bonds in said natural oligonucleotide are substituted by sulfur atoms, or
• 3) a methylphosphonate oligonucleotide wherein oxygen atoms in the hydroxyl groups in the phosphodiester bonds in said natural-type oligonucleotide are substituted by methyl groups, and
• said peptide nucleic acid having bases, i.e., purine or pyrimidine, and
• said bases being linked together by peptide bonds to form a 2-aminoethylglycine backbone.