Various lines of evidence indicate that Rar and Rxr mRNAs are already expressed at early developmental stages. In zebrafish, all rar and rxr genes are maternally-expressed; expression appears to be diffuse at the cleavage and blastula stages, with the first signs of localized expression being seen at the gastrula stage [Hale et al., 2006; Tallafuss et al., 2006; Waxman and Yelon, 2007]. Currently, four Rar genes (raraa, rarab, rarga, rargb), and four Rxr genes (rxra, rxrba, rxrbb, rxrg), have been characterized in zebrafish. Thus, the zebrafish genome contains duplicated co-orthologs of mammalian Rara, Rxrg and Rxrb, but no identified orthologue to Rarb ([Hale et al., 2006; Tallafuss et al., 2006] and references therein). In Xenopus, Rara and Rarg were reported as the main Rar genes expressed, both maternally and zygotically, at pregastrulation stages [Blumberg et al., 1992; Ellinger-Ziegelbauer and Dreyer, 1991; Ellinger-Ziegelbauer and Dreyer, 1993; Koide et al., 2001; Pfeffer and De Robertis, 1994; Shiotsugu et al., 2004]. There are no specific reports of Rar/Rxr gene expression at pre-implantation stages in mouse or rat embryos. Rara, Rarg, Rxra and Rxrb
