Testing different mutations at the same position which have different effects is a particularly stringent test of the ability of RBFE methods to predict antibiotic resistance. We therefore also tested the gyrA A90S mutation (Fig. ) -this is not seen clinically but a serine is present at the equivalent position in the DNA gyrase of other bacterial species and is suggested to help stabilise the gyrase-fluoroquinolone complex via participation in water-ion bridging interactions with the drug coordinated Mg 2+ . M. tuberculosis has some innate immunity to fluoroquinolones which has been suggested is due to the alanine at this position . The gyrA A90S mutation is therefore expected to strengthen the binding of moxifloxacin, thereby conferring hyper-susceptibility.