Two murine Rar genes (Rara and Rarb) are coexpressed within the developing neural tube by E9.5-10.5, but show differential rostral boundaries in the hindbrain neuroepithelium during its transient segmentation into rhombomeres. Rara is expressed until the rhombomere (r)6/7 boundary, and is additionally expressed in r4 [Ruberte et al., 1991]. Although both Rara1 and Rara2 mRNA isoforms are expressed in the neural tube, it is the Rara2 isoform that shows highest expression and displays a clear rhombomeric boundary [Mollard et al., 2000]. Rarb (principally the Rarb2 isoform) is strongly expressed in the neural tube epithelium, up to the r6/r7 boundary (see Figure 2) and ([Mollard et al., 2000; Ruberte et al., 1991; Serpente et al., 2005]; see [Hale et al., 2006; Waxman and Yelon, 2007] for data on zebrafish rar genes). RA is synthesized by RALDH2 in the occipital somites and the posterior hindbrain mesenchyme, and is thought to act by diffusion towards the hindbrain neuroepithelium (see [Glover et al., 2006; Maden, 2002] for reviews). Hence, the discrete, segmental expression patterns of Rara and Rarb may be critical for proper rhombomeric
