IFNG and neoplasm: The internal causes focus on the inherent characteristics of tumor cells; these include defective tumor immunorecognition, epigenetic regulation, abnormal oncogenic signaling, and IFN-γ signal pathway, while the external causes are mainly emanated from the tumor microenvironment, such as exhaustion of T cells, immunosuppressive cells and cytokines, tumor metabolites, new immune checkpoints, and intestinal microflora (128).