In this study, we found that CD8A expression, a marker of CD8+ T-cell infiltration in tumor, was much more strongly correlated with IDO-1 expression than PD-L1 expression, a marker of response to PD-1 blockade (Khunger et al., 2017); and high tumor mutational burden, exogenous viral infection in tumor, and expression of endogenous retrovirus in tumor were associated with IDO-1 over-expression in most cancer types. This evidence concerns the gene CD8A and viral infectious disease.