Lacking an assay allowing us to directly determine the activity of IDO1 enzyme in tumor tissues, most of the preclinical data provided by these IDO1-inhibition trials were derived from serum concentrations of Trp, Kyn, or the Kyn/Trp ratio, chosen as surrogate indicators for IDO1 activity evaluation [25,26,27,28]. Here, IDO1 is linked to neoplasm.