Figure 2.

Possible mechanisms for the function and evolution of distributed enhancers. A: “Speed limit”: if a single module drives a sub-optimal rate of transcription, multiple modules may be required. B: “Failure rate”: if a given enhancer fails to activate transcription in a fraction of cells, a second, independently acting enhancer may significantly improve fidelity of gene expression. C: Patterning precision: two modules with overlapping patterns, but different regulatory logic, combine to produce a novel, refined gene expression pattern. D: Short-range modular interference: two enhancer modules may require a physical separation to prevent undesirable short-range positive and negative transcription factor interactions between the two modules. E: Evolvability: enhancer redundancy may create opportunities for the generation of novel patterns of gene expression. See text for a fuller discussion of these concepts.