IDO1 and metastatic malignant neoplasm in the lymph nodes: From the point of view of considering the expression of IDO1 as a biomarker, it should be noted that Liu et al. showed an association of IDO1 expression with a poor prognosis of GC (HR = 1.596, 95% CI = 1.156–2.204, p = 0.005), depth of infiltration (p = 0.045) and lymph node metastasis (p < 0.001) [111].