Viral vectors

1. A herpes simplex virus which is modified such that it lacks a functional ICP34.5 encoding gene, which is derived from HSV1 JS1 as deposited at the European collection of cell cultures (ECAAC) under accession number 01010209 and which comprises:

(i) a heterologous gene encoding a prodrug converting enzyme; and

(ii) a heterologous gene encoding a protein capable of causing cell to cell fusion.

2. A virus according to claim 1 , wherein said prodrug converting enzyme is a cytosine deaminase.

3. A virus according to claim 1 , wherein said protein capable of causing cell to cell fusion is a gibbon ape leukaemia fusogenic glycoprotein.

4. A virus according to claim 1 , which further comprises a heterologous gene encoding an a protein capable of enhancing an anti-tumor therapeutic effect of the virus.

5. A virus according to claim 4 , wherein the protein is an immunomodulatory protein selected from group consisting of GM-CSF, TNFα and CD40L.

6. A virus according to claim 1 , which further lacks a functional gene encoding ICP47.

7. A virus according to claim 1 , which further lacks a functional gene encoding ICP6, glycoprotein H and/or thymidine kinase.

8. A virus according to claim 1 , which further lacks a gene encoding a functional protein capable of inhibiting dendritic cell function.

9. A virus according to claim 8 , in which said gene encoding a functional protein capable of inhibiting dendritic cell function is UL43 or vhs.

10. A pharmaceutical composition comprising as active ingredient a virus according to claim 1 and a pharmaceutically acceptable carrier or diluent.

11. A method of treating a tumor in an individual in need thereof by administering to said individual an effective amount of a virus according to claim 1 .

12. A method according to claim 11 wherein said virus is administered by direct intra-tumoral inoculation.