IDO is the only rate‐limiting enzyme outside the liver that catalyzes the catabolism of tryptophan (TRP) along the kynurenine (KYN) pathway and plays an important regulatory role in transplantation immune tolerance, tumour immune escape, maternal‐foetal immune tolerance and autoimmune diseases.4, 5 Tryptophan is an essential amino acid for T cell activation and proliferation. The gene discussed is IDO1; the disease is neoplasm.