In pancreatic and gastric cancer models, 5-HT has been shown to upregulate the expression of PD-L1 through histone serotonylation and subsequent epigenetic regulation of immune checkpoint expression.153 Additionally, the precursor of 5-HT, tryptophan, is metabolized by indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO) into kynurenine, which is involved in neuroactivity and immune modulation and is associated with neurodegenerative diseases and cancer.153. Here, TDO2 is linked to neurodegenerative disease.