Although the inhibitor of IDO1, the l-stereoisomer of 1-methyltryptophan (1-MT), or its d-stereoisomer have been claimed to inhibit IDO2, this is still disputed.47, 48, 49 Our studies with PBMC and the AHR antagonist suggest that the antagonist can attenuate IDO2 expression in PASC patients, likely to restore cell functioning. This evidence concerns the gene IDO1 and long COVID-19.