IDO2 and glioblastoma: While TDO expression is expressed at an even higher mRNA level than IDO in patient-resected GBM (33, 109–111), IDO2 levels are negligible or undetectable at the mRNA level (65), despite an IHC-focused study suggesting high IDO2 protein levels in GBM (n = 52) (109); the latter of which likely reflects conclusions based on non-specific antibody immunostaining.