In 4/6 treated patients, serum concentrations of IL-12 and/or canine IFN-γ were detected in multiple blood samples (all three dogs with mast cell tumors and the dog with pulmonary histiocytic sarcoma), showing that the therapy elicited systemic release of the encoded transgene and an IFN-γ response. This evidence concerns the gene IFNG and mast cell neoplasm.