IDO1 and colorectal cancer: In colorectal cancer cells, overexpression of IDO inhibits the infiltration of immune cells (CD3+ T, CD8+ T, CD3+ CD8+ T, and CD57+ NK cells), leading to immune escape, distant metastasis, consumption of local TRP, and the production of pro-apoptotic factors, significantly promoting disease progression and shortening overall survival of patients57–59.