The 35S promoter from the cauliflower mosaic virus (CaMV) is used frequently to control transgene expression in plants. Now on page 995, Al-Kaff et al. have shown that a 35S promoter-driven transgene encoding herbicide resistance in oilseed rape plants can be downregulated following infection of the plants with CaMV. The silencing of the transgene and herbicide tolerance could be triggered by different CaMV pathogenic variants and at different stages of plant development. Because CaMV is a prevalent pathogen worldwide, these results have implications for the stability of 35S promoter-driven transgene expression in transgenic crops.