Unique events in the genome of Mycobacterium tuberculosis, including deletions and IS6110 insertions, have been proposed to be responsible for the virulence phenotype of outbreak strains. Based on this premise, we determined ten IS6110 insertion sites in the genome of the M. tuberculosis CH strain, which was responsible for a large outbreak in Leicestershire, England. Together with previous data on genomic deletions, it was found that 16 genes were mutated either by IS6110 insertions or deletions. The likely impact of these genomic events on the phenotype of the CH strain is discussed.