Characterization of the most common embCAB gene mutations associated with ethambutol resistance in Mycobacterium tuberculosis isolates from Iran Azar Dokht Khosravi,1,2 Mehrandokht Sirous,2 Mahtab Abdi,2 Nazanin Ahmadkhosravi3 1Infectious and Tropical Diseases Research Center, Health Research Institute, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran; 2Department of Microbiology, Faculty of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran; 3Khuzestan Tuberculosis Regional Reference Laboratory, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran Introduction: Ethambutol (Emb) is one of the first-line drugs in the standard combination therapy for tuberculosis; however, due to the rapid increase in Emb resistance among clinical isolates of Mycobacterium tuberculosis (MTB), early detection of Emb resistance is desirable. As the embCAB operon is considered involved in resistance to Emb, this study aimed to analyze the most common mutations within the embCAB operon among MTB isolates from Iran to find any correlations of these mutations with Emb resistance. Methods: A total of 307 clinical isolates of MTB were screened for Emb resistance by phenotypic drug-susceptibility testing. PCR amplification was performed on extracted DNA from all Emb-resistant and randomly selected Emb-susceptible isolates using sets of primers for various gene loci of embC, embA, and embB, followed by sequencing for the detection of most common alterations. Results: In total, ten isolates showed resistance to Emb by phenotypic susceptibility testing (3.25%). The mutation rate in ten Emb-resistant MTB strains was 20%