We speculate that a rational way to attenuate the pathogenicity of the live wild-type virus would be to swap its two promoters, giving an S prom of (5.4) À1 . For this promoter swap we predict virion production would be decreased by 3.3fold and 14.5-fold for infected BHK and DBT cells, respectively, relative to wild-type. However, the extent of growth attenuation by the promoter swapping can be higher than the model prediction because the swapping may also perturb viral transcription and virion assembly and budding processes modulated by the signals encoded in the 39and 59 termini of the genome [42, 43] that are not yet sufficiently defined to be included in the simulation.