Interestingly, some researchers have found that tryptophan (rather than serine) is the theoretical source of IDO1 (an enzyme in tumor immune escape) metabolism of one-carbon unit, and their research results showed that when cancer cells express IDO1, it will promote tryptophan to generate a carbon unit for the de novo synthesis of purine nucleotides [94]. The gene discussed is IDO1; the disease is neoplasm.