However, OVs are also potent inducers of host immunity, which is increasingly recognised to be the major component of their anti-tumour efficacy.15 Encouraging responses have been observed in late-stage clinical testing without significant toxicity, with the first OV recently licensed for the treatment of melanoma16 (‘T-Vec’, herpes simplex virus type 1 (HSV-1) encoding human Granulocyte macrophage colony stimulating factor (GM-CSF)). The gene discussed is CSF2; the disease is neoplasm.