Many morphogenetic events occur while gastrulation proceeds in the posterior region of the embryo. These include induction and regional patterning of the neurectoderm, migration of cranial neural crest cells and formation of the branchial arches, segmentation of the paraxial mesoderm into somites, fusion and looping of the heart tube, etc. Distinct patterns of expression are observed for all Rar genes during this period, which are summarized hereafter for murine genes (see Figure 1; additional data on zebrafish and Xenopus can be found in [Blumberg et al., 1992; Dreyer and Ellinger-Ziegelbauer, 1996; Ellinger-Ziegelbauer and Dreyer, 1991; Ellinger-Ziegelbauer and Dreyer, 1993; Hale et al., 2006; Koide et al., 2001; Pfeffer and De Robertis, 1994; Waxman and Yelon, 2007]). Rara (Figure 1A-C) expression progressively switches from a diffuse pattern, to a prominent expression within the neural ectoderm, with a discrete rostral expression boundary at the level of the prospective hindbrain (for the sake of clarity, expression features at later stages of head and brain development are detailed in the next section). Rarb (Figure 1D-F) and Rarg (Figure 1G-I) show strikingly distinct distributions along
