An ICP34.5, ICP47-deleted, oncolytic herpes simplex type-1 virus (HSV-1) based on the JS1 strain, and encoding the immunostimulating factor human cytokine granulocyte-macrophage colony stimulating factor (GM-CSF) with potential immunostimulating and antineoplastic activities. Upon intratumoral injection, talimogene laherparepvec selectively infects and replicates in tumor cells, thereby inducing tumor cell lysis. In addition, GM-CSF attracts dendritic cells (DCs) and may stimulate a cytotoxic T cell response against tumor cells, which results in immune-mediated tumor cell death. Deletion of the gene encoding for ICP34.5 provides tumor selectivity and prevents replication in healthy cells. As ICP47 blocks antigen presentation in HSV-infected cells, deletion of this gene may induce a more potent antitumor immune response in the tumor cells. Additionally, deletion of ICP47 causes increased expression of the HSV US11 gene and allows US11 to be expressed as an immediate early and not a late gene. This further enhances the degree of viral replication and oncolysis of tumor cells. or A drug used to treat melanoma that has recurred (come back) after surgery. It is used in patients whose cancer is in the skin and lymph nodes and cannot be removed by surgery. It is also being studied in the treatment of other types of cancer. Talimogene laherparepvec is made with a form of the herpesvirus that has been changed in the laboratory to infect and break down cancer cells without harming normal cells. It may also help the immune system kill cancer cells. Talimogene laherparepvec is injected directly into tumors in the skin and lymph nodes. It is a type of oncolytic virus therapy. Also called Imlygic and T-VEC.