DFR 4 is currently identified in Y pseudotuberculosis and some Y pestis strains (in 0.PE4 and other groups on branch 2). This island is an example of the decay common to the highly plastic Y pestis genome.28 What part DFR 4 and novel non-synonymous SNPs might have played in virulence is unknown. However, additional sequences from ancient plague
pandemics will allow us to assess the virulence capacity of these mutations. In the present report we have focused
exclusively on possible genomic diff erences within our ancient Y pestis strains that might have accounted for the high mortality associated with the fi rst plague pandemic. However, host and environmental factors also probably had a role in determining mortality.