A classical model to study pattern formation is provided by the epidermal sensory organs (bristles and other sensilla) that cover the body of Drosophila. Many of these sensory organs (SOs) arise in very constant positions. How are these positions specified? To a large extent, they are defined by the highly resolved sites of expression of the proneural genes of the achaete-scute complex (AS-C). These genes, which confer to cells the capacity to become SO precursors, attain their resolved patterns of expression by means of many position-specific enhancers located within the non-transcribed AS-C DNA. Each enhancer drives expression at one or very few sites. Evidence is growing that the enhancers interact with combinations of activators and repressors (prepattern) distributed in partially overlapping domains which are larger than the AS-C expressing sites. AS-C transcription is activated only at sites with appropriate combinations of factors. Thus, the AS-C integrates the positional information embodied in the relatively broad distributions of prepattern factors and creates a sharper and topographically more precise pattern.