IDO1 and nasopharyngeal carcinoma: These results are consistent with and substantially extend the finding of a recent study (Feng et al., 2020) that found IDO-1 expression to be correlated with tumor mutational burden in breast and cervical cancer, and T-cell infiltration in breast and gynecologic cancers; and previous studies that reported IDO-1 over-expression due to exogenous viral infection in EBV+ nasopharyngeal cancer (Liu et al., 2014), HPV+ cervical (Kobayashi et al., 2008) and head-neck squamous cell cancer (Sailer et al., 2019), and chronic HCV infected liver (Larrea et al., 2007).