- The symptoms of an increased IDO activity (resulting in a depletion of tryptophan and generation of kynurenine), are similar to some ME/CFS symptoms. An interventional study showed that the mechanism might play a more important role in the initiation of the disease than during the established disease (26).- Genes of IDO isoforms have been found to be mutated. Therefore a different hypothesis is, that mutations in IDO result in the opposite with low kynurenine levels and an accumulation of tryptophan, leading to the typical pathological steady state and clinical presentation of ME/CFS (27). The gene discussed is IDO1; the disease is myalgic encephalomeyelitis/chronic fatigue syndrome.