We also found that 3-hydroxy-dl-kynurenine, which his produced from kynurenine by the enzyme kynurenine 3-hydroxylase, and 5-hydroxy-dl-tryptophan, which is produced from tryptophan by the enzyme tryptophan 5-monooxygenase, were increased, correlating with the severity of COVID-19 (Fig. 3B). This evidence concerns the gene KMO and COVID-19.