In Escherichia coli, DNA proofreading is performed by the dnaQ-encoded 3′–5′ exonuclease; however, a new study now shows that the dnaQ homologue in Mycobacterium tuberculosis does not have a role in replication fidelity. Instead, the polymerase and histidinol phosphatase (PHP) domain of the M. tuberculosis replicative polymerase DnaE1 has intrinsic 3′–5′ exonuclease activity and is capable of correcting mismatches. Mutant strains in which the PHP domain of DnaE1 was inactivated exhibited growth defects and mutation rates that were increased by ~3000-fold. Furthermore, the authors report that most bacterial replicative polymerases contain an active PHP exonuclease, which suggests that the PHP domain is the most common replicative exonuclease in the bacterial kingdom and might be an ancestral prokaryotic proofreader. Finally, inactivation of the PHP domain renders mycobacteria sensitive to nucleoside analogues, possibly providing a new strategy for the treatment of drug-resistant M. tuberculosis.