Genes involved in the first steps of pterin cofactor (a component of the molybdenum cofactor) biosynthesis are known to be expanded in the Mtb complex [50] . Molybdenum cofactor-requiring enzymes (such as xanthine oxidase and aldehyde oxidase) could have physiological functions in the metabolism of reactive oxygen species during stress response [51] . Molybdenum cofactor is an efficient catalyst in oxygen-transfer reactions, can be used in anaerobic respiration, and can catalyze redox reactions in carbon, nitrogen, and sulfur metabolism. Recently, genes related to molybdenum cofactor protein synthesis have been shown to be upregulated under conditions of stress in Mtb [52] . Molybdenum cofactor biosynthesis has been previously linked to pathogenesis. The regulator of the moa1 locus, MoaR1, was identified as having a SNP in M. bovis BCG, but not in virulent M. bovis or Mtb [53] . In addition, moa3 is present with varying frequency in the RD1 region, which is absent in M. bovis BCG, of pathogenic strains [54] .