Plants are able to respond to pathogen attack to restrain development of a systemic infection. The response of Brassica napus (oilseed rape) to systemic infection with the DNA virus cauliflower mosaic virus was shown to result in enhancement and subsequent suppression of viral gene expression in parallel with changes in symptom expression, Transgenes with homology to viral sequences were also affected, This phenomenon, which was shown to be mediated by both transcriptional and posttranscriptional mechanisms, might be related to regulation of highly expressed genetic elements.