Arrows in Figure 2 indicate the C T value for each of the four targets examined. Inoculation with 1 particle cell gave no detectable increase in transcript abundance but an increase was observed using 10 particles per cell and the dose response curves strongly suggest that lower doses have an effect. Interpreting these data in the context of infection is not straightforward as particle:infectivity ratios for herpes simplex virus are, at best, 20 or more. On this basis a single infectious unit per cell is clearly sufficient to increase transcript abundance, indicating that signalling occurs at 'physiological' multiplicities of infection.