The most advanced VV immunotherapeutic agent, Pexa-Vec (JX-594), has been engineered by deleting the viral gene that encodes thymidine kinase (TK) for tumor selectivity through insertion and expression of the immuno-stimulating gene that encodes granulocyte/macrophage-colony-stimulating factor (GM-CSF), to enhance antitumor immune responses via dendritic cell activation and the subsequent stimulation of T-lymphocyte activity [3]. Here, TKT is linked to neoplasm.