As a consequence, upregulation of IDO1 expression in tumor cells, programmed cell death 1 (PD-1) in CD8+ T cells and forkhead box P3 (FOXP3) in CD4+ T cells occur, overall impairing CD8+ T cells proliferation and promoting the conversion of CD4+ T cells to Tregs (27–29). This evidence concerns the gene IDO1 and neoplasm.