Hox genes in vertebrates are clustered, and the organization of the clusters has been highly conserved during evolution. The conservation of Hox clusters has been attributed to enhancers located within and outside the Hox clusters that are essential for the coordinated ''temporal'' and ''spatial'' expression patterns of Hox genes in developing embryos. To identify evolutionarily conserved regulatory elements within and outside the Hox clusters, we obtained contiguous sequences for the conserved syntenic blocks from the seven Hox loci in fugu and carried out a systematic search for conserved noncoding sequences (CNS) in the human, mouse, and fugu Hox loci. Our analysis has uncovered unusually large conserved syntenic blocks at the HoxA and HoxD loci. conserved noncoding sequences ͉ conserved synteny ͉ fugu ͉ global control region H ox genes code for homeodomain-containing transcription factors that determine the anterior-posterior patterning of tissues along the body axis of animals. In vertebrates, Hox genes are organized into tight clusters containing up to 14 paralogs. A remarkable feature of these clusters is that the positions of genes in the cluster are colinear with their spatial and temporal expression pattern along the anterior-posterior axis of the embryo. Hox genes at the 3Ј end of the cluster (see Fig. 1) express in the anterior segments of the embryo and are turned on relatively early during development, whereas genes at the 5Ј end express in the posterior segments and at a later stage during development (1). More than two decades after the discovery of Hox clusters, the molecular mechanisms underlying this orchestrated expression pattern of Hox genes and the selective pressure that has maintained the clustering and colinear organization of vertebrate Hox genes still are not well understood.The colinear organization of Hox genes has been shown to be essential for the temporal progression of expression but not for the spatial expression patterns. Transgenic Hox genes inserted at ectopic locations in the genome retain spatial expression to a large extent but fail to recapitulate their temporal pattern of regulation (1). The mechanism of colinear expression of vertebrate Hox genes may require coordinated alterations in the higher-order chromatin structure from one end of the cluster to the other (2). Such a mechanism could require the clustering and colinear organization of Hox genes to be strictly maintained.Another factor that may have contributed to the clustering and colinearity of Hox genes is the existence of global enhancers that regulate several genes in a manner independent of their local enhancers but dependent on their location in the cluster. Global control regions (GCRs) have been identified on either side of the HoxD cluster. The 5Ј HoxD GCR, regulating the expression of Lnp, Evx2, and posterior HoxD genes in developing digits and the central nervous system, is located Ϸ240 kb upstream of HoxD13 in the intergenic region between Atp5g3 and Lnp (3). The 3Ј HoxD GCR mapped downstream of HoxD1...