Methods and products for expression of micro RNAs

1. A precursor microRNA molecule, comprising:

an isolated nucleic acid comprising:

a stem-loop structure, wherein a microRNA sequence is incorporated into a stem of the stem-loop structure, and

a microRNA flanking sequence flanking each end of the stem-loop structure,

wherein each microRNA flanking sequence comprises the sequence of a naturally occurring microRNA flanking sequence at least 40 nucleotides long, wherein the naturally occurring microRNA flanking sequence comprises at least 40 nucleotides of genomic sequence that, when present within a genome, is directly adjacent to the genomic sequence of a naturally occurring microRNA-containing stem-loop structure, and wherein the microRNA sequence and the microRNA flanking sequences are not derived from the same microRNA gene.

2. The precursor microRNA molecule of claim 1 , wherein each microRNA flanking sequence is between 40 and 2,000 nucleotides in length.

3. The precursor microRNA molecule of claim 1 , wherein the microRNA sequence is an artificial microRNA sequence.

4. The precursor microRNA molecule of claim 1 , wherein the precursor microRNA molecule includes at least two stem-loop structures.

5. The precursor microRNA molecule of claim 1 , wherein the molecule has the following nucleic acid sequence

wherein X 1 and X 2 are nucleotides and wherein N 1 and N 2 are nucleic acids of 16-28 nucleotides in length and wherein N 1 and N 2 have at least partial complementarity.

6. The precursor microRNA molecule of claim 5 , wherein X 1 and X 2 are each between 40 and 4,000 nucleotides in length.

7. The precursor microRNA molecule of claim 1 , wherein each microRNA flanking sequence comprises at least 60 nucleotides of a naturally occurring microRNA flanking sequence.

8. The precursor microRNA molecule of claim 1 , wherein each microRNA flanking sequence is between 40 and 150 nucleotides in length.

9. The precursor microRNA molecule of claim 1 , wherein each microRNA flanking sequence is between 40 and 270 nucleotides in length.

10. A precursor microRNA molecule, comprising:

nucleic acid comprising:

a stem-loop structure, wherein a microRNA sequence is incorporated into a stem of the stem-loop structure, and,

a microRNA flanking sequence flanking each end of the stem-loop structure,

wherein the microRNA sequence and the microRNA flanking sequences are not derived from the same microRNA gene, and wherein each microRNA flanking sequence comprises the sequence of a naturally occurring microRNA flanking sequence at least 40 nucleotides long, wherein the naturally occurring microRNA flanking sequence comprises at least 40 nucleotides of genomic sequence that, when present within a genome is directly adjacent to the genomic sequence of a naturally occurring microRNA-containing stem-loop structure.

11. The precursor microRNA molecule of claim 10 , wherein each microRNA flanking sequence is between 40 and 2,000 nucleotides in length.

12. The precursor microRNA molecule of claim 10 , wherein each microRNA flanking sequence is between 40 and 4,000 nucleotides in length.

13. The precursor microRNA molecule of claim 10 , wherein each microRNA flanking sequence is between 40 and 150 nucleotides in length.

14. The precursor microRNA molecule of claim 10 , wherein each microRNA flanking sequence is between 40 and 270 nucleotides in length.

15. A vector for producing a precursor microRNA, wherein the vector includes (i) a sequence encoding a precursor microRNA, including a microRNA sequence incorporated into a stem of a stem-loop structure and a microRNA flanking sequence flanking each end of the stem-loop structure, and (ii) at least one promoter element, wherein each microRNA flanking sequence comprises a naturally occurring microRNA flanking sequence at least 40 nucleotides long, wherein the naturally occurring microRNA flanking sequence comprises at least 40 nucleotides of genomic sequence that, when present within a genome, is directly adjacent to the genomic sequence of a naturally occurring microRNA-containing stem-loop structure.

16. The vector of claim 15 , wherein the vector is a viral vector.

17. The vector of claim 16 , wherein the viral vector is a retroviral vector.

18. The vector of claim 15 , wherein the at least one promoter is an inducible promoter.

19. The vector of claim 15 , wherein the precursor microRNA molecule is a precursor microRNA molecule comprising a nucleic acid comprising:

a stem-loop structure, wherein a microRNA sequence is incorporated into a stem of the stem-loop structure, and,

a microRNA flanking sequence flanking each end of the stem-loop structure,

wherein the microRNA sequence and the microRNA flanking sequences are not derived from the same microRNA gene, wherein each microRNA flanking sequence comprises the sequence of a naturally occurring microRNA flanking sequence at least 40 nucleotides long, and wherein the naturally occurring microRNA flanking sequence comprises at least 40 nucleotides of genomic sequence that, when present within a genome is directly adjacent to the genomic sequence of a naturally occurring microRNA-containing stem-loop structure.

20. The vector of claim 15 , wherein the vector has the nucleic acid sequence of SEQ ID NO. 1 or a variant thereof, wherein the variant has at least 90% identity with SEQ ID NO: 1.

21. The vector of claim 20 , wherein the vector has the nucleic acid sequence of SEQ ID NO. 1.

22. The vector of claim 15 , wherein the promoter is functional in mammalian cells.

23. The vector of claim 15 , wherein each microRNA flanking sequence comprises a naturally occurring microRNA flanking sequence at least 60 nucleotides long.

24. The vector of claim 15 , wherein the microRNA sequence is a mammalian sequence.

25. The vector of claim 15 , wherein the precursor microRNA is identical to a naturally occurring stem-loop sequence which is a mammalian sequence.

26. The vector of claim 15 , wherein each naturally occurring microRNA flanking sequence is a mammalian sequence.

27. The vector of claim 15 , wherein the microRNA sequence is a human sequence.

28. The vector of claim 15 , wherein the precursor microRNA is identical to a naturally occurring stem-loop sequence which is a human sequence.

29. The vector of claim 15 , wherein each naturally occurring microRNA flanking sequence is a human sequence.

30. The vector of claim 15 , wherein the promoter is a mammalian RNA polymerase II promoter.

31. The vector of claim 15 , wherein the microRNA sequence and the microRNA flanking sequences are not derived from the same microRNA gene.

32. The vector of claim 15 , wherein the microRNA sequence is an artificial microRNA sequence.

33. The vector of claim 15 , wherein each microRNA flanking sequence is between 40 and 150 nucleotides in length.

34. The vector of claim 15 , wherein each microRNA flanking sequence is between 40 and 270 nucleotides in length.

35. A host cell transfected with the vector of claim 15 .

36. A vector for producing a precursor microRNA, wherein the vector includes a sequence encoding a precursor microRNA, wherein the precursor microRNA includes a microRNA sequence incorporated into a stem of a stem-loop structure and a microRNA flanking sequence at least 40 nucleotides long flanking each end of the stem-loop structure, wherein each microRNA flanking sequence comprises at least 40 nucleotides of genomic sequence that, when present within a genome, is directly adjacent to the genomic sequence of a naturally occurring microRNA-containing stem-loop structure, and wherein the microRNA sequence and the microRNA flanking sequences are not derived from the same microRNA gene.