 Talimogene laherparepvec is a conditionally replication competent herpes simplex type-1 virus designed for use in solid tumors. It has been specifically modified to replicate in tumors and to provide a local source of the immune-stimulating cytokine, granulocyte macrophage colony stimulating factor GM-CSF. It is injected directly into cancer tumors and is believed to destroy tumor cells by direct infection of the tumor cells and an enhanced immune response due to the release of tumor antigens and GM-CSF expression. 