Mutation EthAW21R confers co-resistance to protionamide and ethionamide in both Mycobacterium bovis BCG and Mycobacterium tuberculosis H37Rv Julius Mugweru,1–3,* Jianxiong Liu,4,* Gaelle Makafe,1,2 Gift Chiwala,1,2 Bangxing Wang,1 Changwei Wang,1 Xinjie Li,4 Yaoju Tan,4 Wing Wai Yew,5 Shouyong Tan,4 Tianyu Zhang1,2 1State Key Laboratory of Respiratory Disease, Guangzhou Regenerative Medicine and Health Guangdong Laboratory (GRMH-GDL), Guangzhou Institutes of Biomedicine and Health (GIBH), Chinese Academy of Sciences (CAS), Guangzhou, China; 2University of Chinese Academy of Sciences, Beijing, China; 3Department of Biological Sciences, University of Embu, Embu, Kenya; 4State Key Laboratory of Respiratory Disease, Department of Clinical Laboratory, Guangzhou Chest Hospital, Guangzhou, 5Stanley Ho Centre for Emerging Infectious Diseases, The Chinese University of Hong Kong, Hong Kong, China *These authors contributed equally to this work Abstract: Ethionamide (ETA) and prothionamide (PRO) are interchangeably used in tuberculosis (TB) chemotherapy regimens. Subtle discrepancies between biochemical and genetic information on the modes of sensitivity and resistance of isoniazid (INH) and ETA warrants further studies. We report a new mutation – EthAW21R – in Mycobacterium bovis Bacillus Calmette-Guérin that corresponds with co-resistance to both PRO and ETA, which to the best of our knowledge has not been reported before. Our findings suggest that mutation EthAW21R could be used as a marker site for testing PRO and ETA cross-resistance. Keywords: mutation, EthAW21R, isoniazid, co-resistance, thioamides, molecular marker