Methods and compositions for the specific inhibition of gene expression by double-stranded rna

An isolated double stranded nucleic acid comprising:

• a first oligonucleotide strand that is 25-30 nucleotides in length and contains 1-3 DNA bases on the 3′ end of the first oligonucleotide strand;
• and
• a second oligonucleotide strand that is 25-30 nucleotides in length comprising an overhang domain and a primary domain wherein the overhang domain is comprised of modified RNA monomers and the primary domain is capable of hybridizing to the first oligonucleotide strand and is comprised of a combination of unmodified and modified monomers,
• wherein said second oligonucleotide strand is sufficiently complementary to a target RNA along at least 19 nucleotides of said second oligonucleotide strand length to reduce target gene expression when said double stranded nucleic acid is introduced into a mammalian or a mammalian cell.

An isolated double stranded nucleic acid comprising first and second oligonucleotide strands, said first oligonucleotide strand being 25 to 30 nucleotides in length and comprising a 5′ terminus having a 5′ terminal nucleotide and a 3′ terminus having a 3′ terminal nucleotide, referring to said first oligonucleotide 5′ terminal nucleotide as position 1 and successively numbered nucleotides in the direction 5′ to 3′ of said first oligonucleotide at each consecutive residue, wherein said second oligonucleotide strand comprises a 5′ terminus having a 5′ terminal nucleotide and a 3′ terminus having a 3′ terminal nucleotide and said second oligonucleotide is 1-4 residues longer at the second oligonucleotide 3′ terminus than said first oligonucleotide, wherein said first oligonucleotide 3′ terminal nucleotide and said second oligonucleotide 5′ terminal nucleotide are paired to form a paired blunt end, wherein said first oligonucleotide 5′ terminal nucleotide is paired with said second oligonucleotide to form a double stranded nucleic acid comprising a single stranded overhang 1-4 nucleotides in length at said 3′ terminus of said second oligonucleotide strand such that said double stranded nucleic acid also comprises a duplex region of at least 25 nucleotide residues, wherein said second oligonucleotide comprises modified nucleotide residues at each nucleotide of said second oligonucleotide that is paired with said first oligonucleotide positions 1, 3, 5, 7, 9, 11, 13 and 15, and wherein said second oligonucleotide strand comprises a sequence complementary to a target RNA and said isolated double stranded nucleic acid reduces target gene expression when introduced into a mammalian cell.