A method of preventing an unwanted micro-RNA (miRNA) from participating in reverse transcription polymerase chain reactions (RT-PCR), the unwanted miRNA having a 5' end and a 3' end, the method comprising: (a) annealing a complementary region of a blocking nucleic acid to a binding region at a first end of the unwanted miRNA, wherein the first end of the unwanted miRNA is one of the 5' end or the 3' end, the blocking nucleic acid comprising: (i) a 5' end of the blocking nucleic acid and a 3' end of the blocking nucleic acid; (ii) a single-stranded complementary region at one of the 5' end of the blocking nucleic acid or the 3' end of the blocking nucleic acid, that anneals with the binding region of the unwanted miRNA under stringent conditions, wherein the complementary region has a terminal end; (iii) a hairpin loop forming region adjacent to the complementary region, the hairpin loop forming region having a ligative terminal end; and (iv) a first blocking moiety linked to the terminal end of the complementary region, in which said first blocking moiety cannot serve as a substrate for ligases; and (b) introducing an RNA adapter and an RNA ligase under conditions to allow the RNA adapter to ligate to the 5' end of the plurality of miRNAs, but wherein the RNA adapter is prevented from ligating to the unwanted miRNA by the blocking nucleic acid. (a) annealing a complementary region of a blocking nucleic acid to a binding region at a first end of the unwanted miRNA, wherein the first end of the unwanted miRNA is one of the 5' end or the 3' end, the blocking nucleic acid comprising: (i) a 5' end of the blocking nucleic acid and a 3' end of the blocking nucleic acid; (ii) a single-stranded complementary region at one of the 5' end of the blocking nucleic acid or the 3' end of the blocking nucleic acid, that anneals with the binding region of the unwanted miRNA under stringent conditions, wherein the complementary region has a terminal end; (iii) a hairpin loop forming region adjacent to the complementary region, the hairpin loop forming region having a ligative terminal end; and (iv) a first blocking moiety linked to the terminal end of the complementary region, in which said first blocking moiety cannot serve as a substrate for ligases; and (i) a 5' end of the blocking nucleic acid and a 3' end of the blocking nucleic acid; (ii) a single-stranded complementary region at one of the 5' end of the blocking nucleic acid or the 3' end of the blocking nucleic acid, that anneals with the binding region of the unwanted miRNA under stringent conditions, wherein the complementary region has a terminal end; (iii) a hairpin loop forming region adjacent to the complementary region, the hairpin loop forming region having a ligative terminal end; and (iv) a first blocking moiety linked to the terminal end of the complementary region, in which said first blocking moiety cannot serve as a substrate for ligases; and (b) introducing an RNA adapter and an RNA ligase under conditions to allow the RNA adapter to ligate to the 5' end of the plurality of miRNAs, but wherein the RNA adapter is prevented from ligating to the unwanted miRNA by the blocking nucleic acid. <EOS>