IDO1 and cancer: Other recognized markers of cancer immune evasion like a high number of T-regulatory cells, increased expression of cytotoxic T lymphocyte antigen-4 CTLA-4, and increased lymphocytes activation inhibitor indoleamine-pyrrole 2,3-dioxygenase IDO, have been found in canine melanoma and were found to correlate with a worse prognosis [94].