The cauliflower mosaic virus (CaMV) 35S enhancer is able to confer strong constitutive expression in plants. We have previously defined two domains within this enhancer that can confer different tissue-specific expression patterns throughout development. We show here that the upstream domain (B) has a modular organization. It contains at least five subdomains that are able to confer distinct expression patterns when fused to the downstream domain (A). When fused to a minimal promoter only three of the five subdomains give any expression in the early stages of plant development. Comparison of the expression patterns conferred by the subdomains alone, in combination with the downstream domain or in combination with other subdomains provides evidence for synergistic interactions among cis-elements within the 35S enhancer.