IDO1 and metastatic malignant neoplasm in the lymph nodes: In that study, the serum levels and expression of IDO did not correlate with histologic classification, tumor size, lymphatic or venous invasion, or lymph node metastasis but was significantly correlated with the serum level of immunosuppressive acidic protein (IAP), a biomarker of systemic immunodepression, suggesting that increased levels of IDO were induced by changes in immune function as opposed to breast tumor IDO secretion.