Toxin-encoding genes from the bacterium Bacillus thuringiensis (Bt) have been widely used in plant transformation to confer resistance to insect pests. These genes have been placed under the control of various promoters. Some, such as the cauliflower mosaic virus (CaMV) 35S and rice actin promoters, are constitutive promoters, that is, they drive gene expression in most tissues at most stages of plant development. Other promoters are tissue-specific, such as the maize phosphoenolpyruvate carboxylase (PEPC) promoter, which is active only in tissues that are photosynthetically active, and the maize tryptophan synthase alpha subunit promoter, which is active in pith. To achieve adequate and sustainable insect control with Bt genes, the associated promoter should provide strong gene expression in tissues attacked by target pests during all stages of plant development that are attacked by the pests (Cohen et al 2000). In some cases, it may also be useful to avoid toxin gene expression in tissues not attacked by target pests, such as roots or grain.