Use of blocker oligonucleotides in selective amplification of target sequences

A method for the selective amplification of at least one target nucleic acid sequence from a sample, the method comprising the ordered steps of:

• (a) treating a sample comprising a target nucleic acid with a tagged oligonucleotide comprising first and second regions, the first region comprising a target hybridizing sequence which hybridizes to a 3′-end of a target nucleic acid sequence contained in the target nucleic acid and the second region comprising a tag sequence situated 5′ to the target hybridizing sequence, wherein the second region does not stably hybridize to the target nucleic acid;
• (b) reducing in the sample the effective concentration of unhybridized tagged oligonucleotide having an active form in which a target hybridizing sequence of the unhybridized tagged oligonucleotide is available for hybridization to the target nucleic acid;
• (c) providing a blocker oligonucleotide to the sample under conditions permitting the blocker oligonucleotide to stably hybridize to a target region of a contaminating nucleic acid that is introduced into the sample after step (b), wherein the contaminating nucleic acid contains the target nucleic acid sequence, wherein the target region is situated 3′ to and spaced no more than 10 bases from the 3′-end of the target nucleic acid sequence contained in the contaminating nucleic acid, and wherein the blocker oligonucleotide is modified to prevent the initiation of DNA synthesis therefrom;
• and
• (d) producing amplification products in a nucleic acid amplification reaction using first and second oligonucleotides, wherein the first oligonucleotide comprises a hybridizing sequence which hybridizes to a 3′-end of the complement of the target nucleic acid sequence and the second oligonucleotide comprises a hybridizing sequence which hybridizes to the complement of the tag sequence, wherein the second oligonucleotide does stably hybridize to the target nucleic acid, and wherein each of the amplification products comprises a base sequence which is substantially identical or complementary to the base sequence of the target nucleic acid sequence and further comprises a base sequence which is substantially identical or complementary to all or a portion of the tag sequence.