Therapeutic compositions

1. An iRNA duplex agent comprising a first and a second sequence, having a first monomer in the first sequence and a second monomer in the second sequence within the first 3, 4, 5, or 6 positions from either the 3′ end or the 5′ end, wherein the first and second monomers are selected such that

i) the first and second monomers are naturally occurring ribonucleotides or modified ribonucleotides having naturally occurring bases, wherein the ribonucleotide of the first monomer and the ribonucleotide of the second monomers, when occupying complementary sites that do not pair and have no substantial level of H-bonding, form a non canonical Watson-Crick pairing; and

ii) the stability of the pairing of the monomers contributing to forming a duplex between the first and second sequence differs from the stability of the pairing between the first sequence and a target sequence or between the second sequence and a target sequence,

wherein the iRNA agent comprises a TT overhang sequence.

2. The iRNA duplex agent of claim 1 , wherein the ribonucleotides form a non canonical Watson-Crick pairing and the non canonical Watson-Crick pairing is selected from the group consisting of U-U, G-G, G-Atrans, G-Acis and G-U.

3. The iRNA duplex agent of claim 2 , wherein the non canonical Watson-Crick pairing is G-U.

4. The iRNA duplex agent of claim 1 , wherein the pairing of the monomers contributes to a duplex between the first and second sequence having decreased stability.

5. The iRNA duplex agent of claim 1 , further comprising at least one mismatch.

6. The iRNA duplex agent of claim 1 , wherein the monomer is selected from 2-amino A, 2-thio U, or 2-thio T.

7. The iRNA duplex agent of claim 1 , wherein the first sequence is an anti-sense strand and the second sequence is a sense strand, wherein the monomer at the selected site in the anti-sense stand includes a U or a modified base which pairs with A, the corresponding monomer in the target is an A, and the sense strand is a monomer which will not pair or which will form a non-canonical pairing.

8. An iRNA duplex agent comprising a first and a second sequence, having a first monomer in the first sequence and a second monomer in the second sequence within the first 3, 4, 5, or 6 positions from either the 3′ end or the 5′ end, wherein the first and second monomers are selected such that

i) the first and second monomers are naturally occurring ribonucleotides or modified ribonucleotides having naturally occurring bases, wherein the ribonucleotide of the first monomer and the ribonucleotide of the second monomers, when occupying complementary sites that do not pair and have no substantial level of H-bonding, form a non canonical Watson-Crick pairing; and

ii) the stability of the pairing of the monomers contributing to forming a duplex between the first and second sequence differs from the stability of the pairing between the first sequence and a target sequence or between the second sequence and a target sequence,

represented by

S

5′ R 1 N 1 N 2 N 3 N 4 N 5 [N] N −5 N −4 N −3 N −2 N −1 R 2 3′

AS

3′ R 3 N 1 N 2 N 3 N 4 N 5 [N] N −5 N −4 N −3 N −2 N −1 R 4 5′

S:AS

P 1 P 2 P 3 P 4 P 5 [N] P −5 P −4 P −3 P −2 P −1 5′

wherein S is the sense strand; AS is the antisense strand; R 1 is an optional 5′ sense strand overhang; R 2 is an optional 3′ sense overhang; R 3 is an optional 3′ antisense overhang; R 4 is an optional 5′ antisense overhang; N is a subunit; [N] indicates that additional subunit pairs may be present; and P x , is a pairing of sense N x and antisense N x , where x is −5 to 5.

9. The iRNA agent of claim 8 , wherein at least 1 or more pairs in P −5 through P −1 are, independently, A:U, G:U, I:C, or mismatched pairs.

10. The iRNA agent of claim 8 , wherein at least 2 of the pairs in P −1 through P −4 , are G:U.

11. The iRNA agent of claim 8 , wherein at least 1 or more pairs in P 5 through P 1 are, independently, G:C, A:T, A:U, 2-amino-A:U, 2-thio U or 5 Me-thio-U:A, G-clamp:G, guanadinium-G-clamp:G; psuedo uridine:A, or a pair in which one or both subunits has a sugar modification.

12. The iRNA agent of claim 8 , wherein least 2 of the pairs in P 1 through P 4 , are G:C.

13. The iRNA agent of claim 8 , wherein at least 1 or more pairs in P −5 through P −1 are, independently, A:U, G:U, I:C, or mismatched pairs, and wherein at least 1 or more pairs in P 5 through P 1 are, independently, G:C, A:T, A:U, 2-amino-A:U, 2-thio U or 5 Me-thio-U:A, G-clamp:G, guanadinium-G-clamp:G; psuedo uridine:A, or a pair in which one or both subunits has a sugar modification.