Method and medicament for inhibiting the expression of a given gene

1 . An oligoribonucleotide of double-stranded structure (dsRNA) for inhibiting the expression of a given target gene in mammalian cells, wherein the dsRNA has 15 to 49 base pairs and has a complementary region I that is incorporated in the dsRNA, and wherein the complementary region I has not more than 49 nucleotide pairs.

2 . The dsRNA of claim 1 , wherein at least one of said strands comprises at least one chemically modified nucleotide.

3 . The dsRNA of claim 2 , wherein said chemically modified nucleotide is a 2′-modified nucleotide.

4 . The dsRNA of claim 3 , wherein said 2′-modified nucleotide is a 2′-methyl substituted nucleotide.

5 . The dsRNA of claim 3 , wherein said 2′-modified nucleotide is a 2′-amino substituted nucleotide.

6 . The dsRNA of claim 3 , wherein said chemically modified nucleotide is a locked nucleotide.

7 . The dsRNA of claim 1 , wherein the target gene is a mammalian gene or a viral gene.

8 . The dsRNA of claim 1 , wherein the target gene is selected from the group consisting of an oncogene, a cytokine gene, an Id protein gene, a developmental gene, a PKR gene and a prion gene.

9 . The dsRNA of claim 1 , wherein inhibiting the expression of said target gene further comprises inhibiting expression at a concentration of the dsRNA that is lower by one order of magnitude than a concentration required for a corresponding single-stranded oligoribonucleotide to inhibit expression.

10 . The dsRNA of claim 1 , wherein the dsRNA is enclosed by a micellar structure.

11 . The dsRNA of claim 10 , wherein the micellar structure comprises a liposome.

12 . The dsRNA of claim 1 , wherein the target gene is expressed in eukaryotic cells.

13 . The dsRNA of claim 1 , wherein the ends of the dsRNA are modified in order for the double-stranded structure to counteract degradation.

14 . The dsRNA of claim 1 , wherein at least two dsRNAs which differ from each other are introduced into the cell.