IDO1 and viral infectious disease: CD8A expression showed a comparable level of correlation with IDO-2 expression and PD-L1 expression, and while high tumor mutational burden and exogenous viral infection in tumor were associated with IDO-2 over-expression in only a minority of cancer types, expression of endogenous retrovirus in tumor was correlated with IDO-2 expression even more strongly than IDO-1 expression.