The presence of various immunosuppressive leukocyte populations, such as tumor-associated macrophages (TAM), tumor-associated neutrophils (TAN), myeloid-derived suppressor cells (MDSC), and regulatory T cells (Tregs), which produce TGF-β, PGE2, indoleamine-2,3 dioxygenase (IDO), reactive oxygen/nitrogen species, tryptophan-2,3 dioxygenase (TDO), and arginase, hindering the recruitment and activation of cytotoxic T cells, is a significant limitation of CAR T treatment of solid tumors. This evidence concerns the gene IDO1 and neoplasm.