IDO1 and neoplasm: In a meta-analysis by Wang et al., high expression of IDO in tumor tissues was associated with poor prognosis (pooled hazard ratio (HR) 1.92, 95% CI, 1.52–2.43, p < 0.001) and tumor progression (pooled HR = 2.25, 95% CI, 1.58–3.22, p < 0.001) in cancer patients [135].