Therapeutic forms of HSV-1 are created by modifying or deleting genes that are crucial for viral replication in normal cells but not in tumor cells, such as thymidine kinase (TK), ICP34.5 (required for viral replication in nerve cells), ICP6 (encoding the large subunit of HSV-1 ribonucleotide reductase), and ICP4786, 87. The gene discussed is TKT; the disease is neoplasm.