A method for preparing a tagging oligonucleotide comprising a targeting portion comprising a hybridizable-complementary nucleotide sequence to a target nucleic acid sequence and a tagging portion comprising a non-hybridizable-non-complementary nucleotide sequence to the target nucleic acid sequence, comprising: (a) selecting the hybridizable-complementary nucleotide sequence to the target nucleic acid sequence for the targeting portion and the non-hybridizable-non-complementary nucleotide sequence to the target nucleic acid sequence for the tagging portion; wherein the tagging portion comprises a first tagging part of 3-8 nucleotides in length adjacent to the targeting portion and a second tagging part of 4-40 nucleotides in length adjacent to the first tagging part; the non-hybridizable-non-complementary nucleotide sequence for the tagging portion is selected not to be hybridized with the target nucleic acid sequence; wherein a non-hybridizable-non-complementary nucleotide sequence of the first tagging part is selected by an independent non-complementarity level such that a sequence with a non-complementarity level satisfying a predetermined threshold value criterion is selected as the non-hybridizable-non-complementary nucleotide sequence of the first tagging part; wherein the non-complementarity level is evaluated by unfavorability between nucleotide bases and the non-complementarity level is scores given depending on unfavorability of the bonds between nucleotide bases A, T, G and C; and (b) preparing the tagging oligonucleotide comprising (i) the targeting portion comprising the selected hybridizable-complementary nucleotide sequence and (ii) the tagging portion comprising the selected non-hybridizable-non-complementary nucleotide sequence. (a) selecting the hybridizable-complementary nucleotide sequence to the target nucleic acid sequence for the targeting portion and the non-hybridizable-non-complementary nucleotide sequence to the target nucleic acid sequence for the tagging portion; wherein the tagging portion comprises a first tagging part of 3-8 nucleotides in length adjacent to the targeting portion and a second tagging part of 4-40 nucleotides in length adjacent to the first tagging part; the non-hybridizable-non-complementary nucleotide sequence for the tagging portion is selected not to be hybridized with the target nucleic acid sequence; wherein a non-hybridizable-non-complementary nucleotide sequence of the first tagging part is selected by an independent non-complementarity level such that a sequence with a non-complementarity level satisfying a predetermined threshold value criterion is selected as the non-hybridizable-non-complementary nucleotide sequence of the first tagging part; wherein the non-complementarity level is evaluated by unfavorability between nucleotide bases and the non-complementarity level is scores given depending on unfavorability of the bonds between nucleotide bases A, T, G and C; and (b) preparing the tagging oligonucleotide comprising (i) the targeting portion comprising the selected hybridizable-complementary nucleotide sequence and (ii) the tagging portion comprising the selected non-hybridizable-non-complementary nucleotide sequence. <EOS>