The FMDV fidelity variants exhibit reduced fitness in vitro. The mutant O-D5N, O-DA, O-DAMM, T64I and AMMR was mixed with the WT at a ratio of 9:1, 1:1 or 1:9 to inoculate BHK-21 cells at an MOI of 0.1 for three passages, after which the 3Dpol region was sequenced. The abundance of each competitor was measured as the height of the nucleotide peak for the mutant (A or T nt) or the WT (G or C nt) in sequencing chromatograms.