Van Laarhoven et al. next examined CSF tryptophan in a validation cohort including 101 TBM cases and 17 controls, and identified indoleamine 2,3-dioxygenase—IDO 1 is an enzyme that catalyzes the first rate-limiting step in tryptophan catabolism—as showing greater expression in patients with TBM than in patients with brain trauma. Here, IDO2 is linked to meningeal tuberculosis.