A compendium of studies have since shown that IDO expression is highly induced in the human TB granuloma environment, and products of IDO-mediated Trp catabolism are detected in the plasma, sera, and urine of patients with active TB, including multidrug-resistant tuberculosis (MDR-TB) as well as TB/HIV, in cohorts from various regions of the world (18–20). Here, IDO1 is linked to multidrug-resistant tuberculosis.