CD8+ T-cell-mediated IDO expression via IFNγ in immunologically “hot” tumors could be one of the explanations why studies in breast cancer (123, 124), gastric adenocarcinoma (125), hepatocellular (126), pancreatic cancer (127), adenosquamous lung carcinoma (128), and prostate cancer (129) observed a positive prognostic effect for tumoral IDO expression. This evidence concerns the gene IFNG and breast cancer.