Imlygic is an oncolytic viral therapy with attenuated life herpes simplex virus type 1 (HSV-1). In HSV-1, two genes are removed and one gene is added. The genes who are removed originally code for the proteins infected cell protein 34.5 (ICP34.5) and infected cell protein 47 (ICP47). ICP34.5 blocks the response of healthy cells to stop replicating and die after viral infection. However, cancer cells lack this system. By removing ICP34.5, HSV-1 cannot replicate and kill normal cells, but in cancer cells they can. ICP47 suppresses an immune response to viral infection. Removing ICP47 should trigger an immune response, in theory. A gene coding for granulocyte colony-stimulating factor (GM-CSF) is inserted to promote an immune response to cancer cells infected by Imlygic. Despite these efforts, clinical evidence for an immune response to cancer cells due to Imlygic is not available.