Real-time PCR confirmed that changes in transcription associated with the NF-kB, JAK/STAT, JAK/Src and PI3K pathways were modulated as a result of virion binding, all of which required gD on the envelope surface To demonstrate that signalling occurred at physiologically relevant multiplicities of infection, HFFs were inoculated with either 1000, 100, 10 or 1 particles per cell of entry-defective HSV-1. Changes in gene transcription occurred in a dose-dependent manner and were detectable at 10 virus particles per cell. Given the particle:infectivity ratios usually quoted for HSV1, we argue that this corresponds to physiological conditions (i.e. less than one infectious unit per cell), but it is impossible to know the circumstances that pertain in vivo when a single cell becomes infected. What seems almost certain is that an infected cell will normally present an uninfected neighbouring cell with 10 or more progeny particles.