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Ethionamide (ETH) is an antibiotic used for the treatment of multidrug-resistant (MDR) tuberculosis (TB) (MDR-TB), and its use may be limited with the emergence of resistance in the Mycobacterium tuberculosis population. ETH resistance in M. tuberculosis is phenomenon independent or cross related when accompanied with isoniazid (INH) resistance. In most cases, resistance to INH and ETH is explained by mutations in the inhA promoter and in the following genes: katG, ethA, ethR, mshA, ndh, and inhA. We sequenced the above genes in 64 M. tuberculosis isolates (n ؍ 57 ETH-resistant MDR-TB isolates; n ؍ 3 ETH-susceptible MDR-TB isolates; and n ؍ 4 fully susceptible isolates). Each isolate was tested for susceptibility to first-and second-line drugs using the agar proportion method. Mutations were observed in ETH-resistant MDR-TB isolates at the following rates: 100% in katG, 72% in ethA, 45.6% in mshA, 8.7% in ndh, and 33.3% in inhA or its promoter. Of the three ETH-susceptible MDR-TB isolates, all showed mutations in katG; one had a mutation in ethA, and another, in mshA and inhA. Finally, of the four fully susceptible isolates, two showed no detectable mutation in the studied genes, and two had mutations in mshA gene unrelated to the resistance. Mutations not previously reported were found in the ethA, mshA, katG, and ndh genes. The concordance between the phenotypic susceptibility testing to INH and ETH and the sequencing was 1 and 0.45, respectively. Among isolates exhibiting INH resistance, the high frequency of independent resistance and cross-resistance with ETH in the M. tuberculosis isolates suggests the need to confirm the susceptibility to ETH before considering it in the treatment of patients with MDR-TB. E thionamide (ETH), a structural analog of isoniazid (INH), is a second-line drug used in the treatment of multidrug-resistant tuberculosis (MDR-TB) (1). Both ETH and INH are classified as prodrugs that are activated by different mycobacterial enzymes. INH is activated by the katG-encoded catalase-peroxidase, and ETH is activated by the ethA-encoded monooxygenase (2, 3). The activated INH and ETH drugs share the same molecular target, i.e., the NADH-dependent enoyl-acyl carrier protein reductase InhA, which is involved in the long-chain mycolic acid biosynthesis pathway (4). Therefore, the cross-resistance between INH and ETH can be detected in Mycobacterium tuberculosis clinical isolates in the case of mutations affecting the common target, which may occur when patients have previously been treated with INH and not with ETH (5). The frequency of cross-resistance differs between countries: 100% in Korea (6), 95.12% in Argentina (7), 94% in Brazil (8), 62% in France (9), and 13.8% in Thailand (10).Resistance to INH and ETH is mainly due to the chromosomal mutations. The mutation-carrying genes, such as those encoding the enzymes KatG (11,12) and EthA (13,14), are associated with individual resistance to INH and ETH, respectively. Mutations at the inhA promoter region or inhA gene result in the ove...