Numerous preclinical studies on metastatic cancers have shown that accelerated tumor growth and enhanced resistance to immunotherapy are related to increased levels of indoleamine 2,3-dioxygenase 1 and arginase 1 in tumor-infiltrating dendritic cells and many other immunosuppressive enzymes/surface molecules produced by tumor-infiltrating cells (including IL-10, coinhibitory receptors, and other failure markers). This evidence concerns the gene IDO1 and neoplasm.