T-VEC is a herpes simplex virus type 1 (HSV-1), in which the human granulocyte-macrophage colony-stimulating factor (GM-CSF) gene was inserted, and the infected cell protein (ICP)34.5 and ICP47 genes were deleted. The insertion of GM-CSF increases immune recognition by provoking anti-cancer immunity, and the deletion of the ICP34.5 and ICP47 genes improves safety, allows for tumour-specific replication, and provides immune evasion functions. The gene discussed is CSF2; the disease is cancer.