Metabolomic profiles of synovial fluid from RA patients revealed the downregulation of tryptophan metabolites, which is attributed to the aberrant methylation of the cytotoxic T lymphocyte antigen 4 gene promoter and a hypoxic environment in synovial fluid leading to indoleamine 2,3-dioxygenase 1 (IDO1) low expression (Kaul et al., 2020; Panfili et al., 2020). Here, IDO1 is linked to rheumatoid arthritis.