High mutational burden in skin melanoma, exogenous viral infection in gastric (EBV) and head-neck squamous cell (HPV) cancers, and ERV3-2 expression in ER+ HER2− breast, gastric, lung, prostate, cervical, and head-neck squamous cell cancers are associated with over-expression of IDO-1 and IDO-2 (but not TDO-2), but it is unclear whether that is sufficient for response to IDO-1 inhibitors. This evidence concerns the gene ESR1 and viral infectious disease.