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Background. Highly lethal outbreaks of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis are increasing. Whole-genome sequencing of KwaZulu-Natal MDR and XDR outbreak strains prevalent in human immunodeficiency virus (HIV)-infected patients by the Broad Institute identified 22 novel mutations which were unique to the XDR genome or shared only by the MDR and XDR genomes and not already known to be associated with drug resistance. Methods. We studied the 12 novel mutations which were not located in highly-repetitive genes to identify mutations that were truly associated with drug resistance or were likely to confer a specific fitness advantage. Results. None of these mutations could be found in a phylogenetically and geographically diverse set of drugresistant and drug-susceptible Mycobacterium tuberculosis isolates, suggesting that these mutations are unique to the KZN clone. Examination of the 600-basepair region flanking each mutation revealed 26 new mutations. We searched for a convergent evolutionary signal in the new mutations for evidence that they emerged under selective pressure, consistent with increased fitness. However, all but 1 rare mutation were monophyletic, indicating that the mutations were markers of strain phylogeny rather than fitness or drug resistance. Conclusions. Our results suggest that virulent XDR tuberculosis in immunocompromised HIV-infected patients can evolve without generalizable fitness changes or other XDR-specific mutations. [ABSTRACT FROM AUTHOR]