Estimated mortality from the Justinian Plague was up to 40%, which might mean that the strains of Y pestis associated with this pandemic were of intrinsically high virulence. Of the 66 unique chromosomal and plasmid SNPs identified A120 and A76 combined, 23 were non-synonymous, two of which are identified in well characterised virulence factors—ail and yopJ. Ail is an outer membrane protein that promotes bacterial resistance to complement-mediated killing and adhesion to host cells. A novel Arg→Gly aminoacid substitution occurs in an extracellular region (loop 3) of the Ail protein from the Justinian strains,
which could potentially increase resistance to complement-mediated killing or adherence, or both.