Retroviral vectors

1. A double-stranded nucleic acid molecule comprising:

(a) a first nucleic acid comprising an env gene; and

(b) a second nucleic acid comprising a gag-pol gene;

wherein coding sequences of the first and second nucleic acids are on opposing strands of the double-stranded nucleic acid molecule, wherein the env and gag-pol genes are independently operably-associated with first and second inducible promoters, respectively, the first and second promoters having less than 95% nucleotide sequence identity between them, and wherein the double-stranded nucleic acid molecule comprises 1 or more nucleotide sequences encoding apoptosis inhibitors IAP1 and EBNA5.

2. The double-stranded nucleic acid molecule as claimed in claim 1 , wherein the nucleotide sequence identity between the first and second promoters is less than 90%, 85%, 80%, 70% or 60%.

3. The double-stranded nucleic acid molecule as claimed in claim 1 , wherein the env gene is a VSV-G gene.

4. The double-stranded nucleic acid molecule as claimed in claim 1 , wherein the gag-pol gene is from HIV-1, or is a derivative thereof.

5. The double-stranded nucleic acid molecule as claimed in claim 1 , wherein the nucleic acid molecule additionally comprises a rev gene.

6. The double-stranded nucleic acid molecule as claimed in claim 1 , wherein the nucleic acid molecule additionally comprises a transgene.

7. A retroviral vector comprising a double-stranded nucleic acid molecule as claimed in claim 1 .

8. A mammalian cell comprising a double-stranded nucleic acid as claimed in claim 1 .

9. A retroviral packaging cell comprising a double-stranded nucleic acid molecule as claimed in claim 1 .

10. A kit comprising:

(i) a retroviral packaging vector comprising a double-stranded nucleic acid molecule as claimed in claim 1 ;

together with one or more of the following:

(ii) a retroviral Transfer Vector comprising a transgene and a retroviral rev gene;

(iii) cells of a cell line suitable for the production of virus particles.

11. A kit comprising:

(i) a retroviral packaging vector comprising a double-stranded nucleic acid molecule as claimed in claim 1 which additionally comprises a rev gene;

together with one or more of the following:

(ii) a retroviral Transfer Vector comprising a transgene;

(iii) cells of a cell line suitable for the production of virus particles.

12. A process for producing a retroviral packaging cell, the process comprising the steps:

(i) stably integrating a double-stranded nucleic acid molecule as claimed in claim 1 into a mammalian cell,

thereby producing a mammalian cell that expresses retroviral env and gag-pol genes, and optionally the rev gene.

13. A process for producing retroviruses, the process comprising the steps:

(a) introducing a retroviral Transfer Vector comprising 5′ and 3′ retrovirus LTRs, a retrovirus packaging signal and a retroviral rev gene into a retroviral packaging cell as claimed in claim 9 , wherein the retroviral packaging cell comprises retroviral env and gag-pol genes stably integrated into its genome;

(b) culturing the cell under conditions such that retroviruses are assembled and secreted by the cell; and

(c) harvesting packaged retrovirus from supernatant.

14. A process for producing retroviruses, the process comprising the steps:

(a) introducing a retroviral Transfer Vector comprising a transgene into a retroviral packaging cell as claimed in claim 9 , wherein the retroviral packaging cell comprises retroviral env, gag-pol and rev genes stably integrated into its genome;

(b) culturing the cell under conditions such that retroviruses are assembled and secreted by the cell; and

(c) harvesting packaged retrovirus from supernatant.

15. A mammalian cell comprising a double-stranded nucleic acid as claimed in claim 8 , wherein the double-stranded nucleic acid is stably integrated into a nuclear genome of the mammalian cell.

16. The retroviral vector as claimed in claim 7 , wherein the retroviral vector is a lentiviral vector.