Expression of Rar genes during later brain development has been studied in most detail in the mouse [Ruberte et al., 1993] and the rat [Zetterstrom et al., 1999]. In both species, Rarg transcripts were not detected in brain structures. Rara and Rarb are expressed in the developing myelencephalon (medulla oblongata), in regions that derive from their early hindbrain expression domains, and expression of Rarb in particular, becomes localized to somatic and visceral motor nuclei [Ruberte et al., 1993]. Rara expression is very low or absent in other brain structures, except in the corpus striatum and pallidum, where the murine Rara2 isoform is expressed from E12.5 onwards in a domain adjacent to that of Rarb (Figure 4A-D) [Mollard et al., 2000; Ruberte et al., 1993]. Both Rarb1 and Rarb2 are coexpressed in the corpus striatum (caudate-putamen and nucleus accumbens) from E12.5 onwards (Figure 4C,D), as well as in the olfactory tubercle [Mollard et al., 2000; Ruberte et al., 1993]. Rarb transcripts are also detected in the choroid plexuses and the developing meninges, which also express the RA-synthesizing enzyme Raldh2 [Niederreither et
