During limb development, coordinated expression of several Hoxd genes is required in presumptive digits. We searched for the underlying control sequences upstream from the cluster and found Lunapark (Lnp), a gene which shares limb and CNS expression specificities with both Hoxd genes and Evx2, another gene located nearby. We used a targeted enhancer-trap approach to identify a DNA segment capable of directing reporter gene expression in both digits and CNS, following Lnp, Evx2, and Hoxd-specific patterns. This DNA region showed an unusual interspecies conservation, including with its pufferfish counterpart. It contains a cluster of global enhancers capable of controlling transcription of several genes unrelated in structure or function, thus defining large regulatory domains. These domains were interrupted in the Ulnaless mutation, a balanced inversion that modified the topography of the locus. We discuss the heuristic value of these results in term of locus specific versus gene-specific regulation.
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