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 the rostro-caudal axis of the embryo. Rarg is expressed in the regressing primitive streak [Ruberte et al., 1991], and during somitogenesis and caudal axis extension its expression is specifically maintained in the caudalmost tissues [Abu-Abed et al., 2003]. Thus, Rarg expression is downregulated, both in the neural plate and in mesodermal derivatives, while these tissues initiate differentiation. Rarb expression is relatively complementary, being undetectable in the caudalmost tissues, and present in the neural tube and some of the mesodermal derivatives at more rostral levels [Ruberte et al., 1991].
