CSF2 and neoplasm: OVs can be broadly categorized into three classes [33,34,35]: (1) natural viruses that can replicate specifically within tumor cells without modification; (2) second-generation OVs achieved through genetic modification, such as deleting viral gene segments, using transcriptional elements as promoters or enhancers, or modifying viral surface proteins; and (3) third-generation OVs created through genetic engineering to express therapeutic genes such as granulocyte-macrophage colony-stimulating factor (GM-CSF).