Rational vaccine attenuation by double genomic manipulations. Several variant VSV strains, including N1 through N4, have been made by Ball and Wertz [18, 20] . With an aim to generate a potentially broader diversity of growth phenotypes, we created and tested in silico VSV mutants by combining N gene relocations with a range of S prom . This is a computationally simple task, but experimentally nontrivial. For example, the VSV N4 with S prom ¼ 0.1 produced a simulated 38-fold fewer virus progeny than wild-type in BHK Figure 6B ). It is interesting to note that this 38-fold degree of growth attenuation is greater than the product of the constituent attenuations (4.7 3 5.4 ¼ 25) that one calculates by assuming that the effects were uncoupled. Such nonmultiplicative effects of double genomic manipulations on growth would be challenging to predict in the absence of a quantitative model.