Segment-specific expression of a homoeobox-containing gene in the mouse hindbrain

THE process of segmentation, in which the developing embryo is divided into repetitive structures along its antero-posterior (A-P) axis, as a means of organizing and coordinating the body plan is found in a wide range of organisms. In Drosophila, homoeotic genes are involved in all levels of segmental organization and in determining segment identity¹. The roles of these genes in seg-mentation have been found mainly by mutational studies^2,3, but also by in situ hybridization, which has shown their domains of expression. In contrast to Drosophila, however, embryonic expression of homoeobox-containing genes in vertebrate organisms has not been found to follow a segmental pattern⁴. Vertebrate segmentation can be clearly seen in the mesodermal somites, but repetitive morphological structures in the central nervous system (neuromeres)^5-13 have only recently been shown to have develop-mental significance. Neuromeres in the hindbrain (rhombomeres) have been defined as segmental units by their pattern of nerve formation in the developing chick¹⁴ and by the alternating expression of Krox-20, a gene encoding a zinc-finger DNA-binding protein, in the 9.5-day-old mouse¹⁵. Here we report that a mouse homoeobox-containing gene, Hox-2.9, is expressed in a segment-specific manner in the developing mouse hindbrain. This expression is in a region which is flanked by the regions of expression of Krox-20, and is precisely contained within a single neuromere, rhombomere 4.