Fig. 7. Summary. (A) Schematic view of a flat-mount hindbrain showing Hox gene expression on the left (bars) and first-order visceral (noradrenergic), somatic and proprioceptive sensory relay interneurons (circles) on the right. The loss of Hoxb1 (B) and the combination of Hoxa3 and Hoxb3 (C) result in a specific loss of visceral interneurons in r4 and r5, respectively. The loss of visceral sensory interneurons in these Hox mutant embryos is associated with the expansion of the somatic sensory interneuron domain. In Hoxa2 loss-of-function (D), somatic sensory interneurons are completely eliminated in r2 and significantly reduced in r3, presumably through the redundant role of Hoxb2 in this rhombomere (see A). Although Hox genes are expressed throughout the early neuroepithelium, the present finding suggests a specific role for Hox genes in the generation of cellular diversity in the developing hindbrain.