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Genes are often activated by enhancers located at large genomic distances, and the importance of this positioning is poorly understood. By relocating promoter-reporter constructs into thousands of alternative positions within a single locus, we dissected the positional relationship between the mouse gene and its distal enhancer. This revealed an intricate, sharply confined activation landscape in which the native gene occupies an optimal position for its activation. Deletion of the gene relaxes this confinement and broadly increases reporter activity. The confining effect of the gene is partially conferred by its ~1-kilobase coding region. Our local relocation approach provides high-resolution functional maps of a genomic locus and reveals that a gene can strongly constrain the realm of influence of its enhancer.