IMLYGIC® (talimogene laherparepvec) has been genetically modified to replicate within tumors and to produce the immune stimulatory protein granulocyte-macrophage colony-stimulating factor (GM-CSF). When IMLYGIC® is injected into melanoma tumors, it causes lysis of tumor cells followed by release of tumor-derived antigens, which together with virally derived GM-CSF may promote an antitumor immune response. However, the exact mechanism of action is unknown.1