We isolated genomic clones containing the mouse Hox 8 gene, a member of the msh gene family. We show that Hox 8 comprises two exons of approximately 600 and 691 bp separated by a 3.5-kb intron, and that it cosegregates with previously mapped markers in the distal region of mouse chromosome 13. In midgestation embryos, the Hox 8 gene produces transcripts of 1.4 and 2.2 kb. Both transcripts are present in facial tissues of the newborn mouse, though the ratio of the 2.2-kb transcript to the 1.4-kb transcript is reduced relative to the ratio observed for midgestation embryos. An alignment of the homeobox sequences of previously characterized members of the msh family revealed three subclasses: Hox 7-like genes, Hox 8 -like genes, and msh-like genes. Both the Hox 7-like genes and Hox 8-like genes are present throughout the vertebrates. Representatives of the third subclass, the msh-like genes, are found in a protostome (Drosophila) and a deuterostome (Ciona) and are thus likely to be phylogenetically widespread. To investigate the distribution of Hox 8 -like genes outside the chordates, we used the polymerase chain reaction and degenerate Hox 8 primers to screen genomic DNA of the purple sea urchin (Strongylocentrotus purpuratus, Phylum Echinodermata). We isolated a gene with greater sequence similarity to mouse Hox 8 than to members of the Hox 7 or msh subfamilies, demonstrating that the Hox 8 subfamily has been in existence at least since the echinoderms diverged from the lineage that gave rise to the chordates. The remarkable sequence conservation of the Hox 8-like homeobox suggests that its target sequence, and, perhaps, other aspects of its function, has remained unchanged for more than 500 million years.