For example, 5-aminolevulinic acid, a precursor of IDO1 enzyme synthesis, can induce the formation of brain edema.41 In the clinic, dexamethasone, a glucocorticoid that downregulates TDO expression,42 is often used as a drug to mitigate patient brain edema.43–45 This study revealed a positive correlation between IDO1/TDO expression and AQP4 expression, and a higher incidence rate of brain edema in the IDO1/TDO-positive group than in the IDO1/TDO-negative group. This evidence concerns the gene TDO2 and brain edema.