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 expression of RA-regulated target genes. These include several Hox genes, including Hoxa1 and Hoxb1 ([Dupe et al., 1997; Studer et al., 1998] and references therein). Combinatorial roles of RARα and RARβ for patterning of posterior rhombomeres have been demonstrated by the analysis of compound knockout mutants ([Dupe et al., 1999b]; see also the accompanying review by [Mark et al., 2009]).
