The expression patterns of RARs and RXRs have been studied in various species. Initial studies, published shortly after the cloning of RARs and RXRs, have been performed on mouse embryos [Dolle et al., 1989; Dolle et al., 1990; Ruberte et al., 1991; Ruberte et al., 1990]. Reports on other species have often focused on early developmental stages (e.g., [Hale et al., 2006; Tallafuss et al., 2006] for recent reports on zebrafish rar and rxr gene expression), or on specific organ systems. In a few instances, expression of specific Rar isoforms has been studied [Koide et al., 2001; Mollard et al., 2000; Smith, 1994]. Most studies have analyzed the Rar and Rxr expression patterns at the mRNA level, using in situ hybridization (ISH) and occasionally RT-PCR (e.g., [Ulven et al., 2000]), but there are some reports of RAR/RXR protein distributions analyzed by immunohistochemistry (e.g., [Mori et al., 2001]). Unless otherwise mentioned, the data reviewed hereafter are mRNA expression data. As a general trend, three of the retinoid receptors (RARα, RXRα and RXRβ) have ubiquitous, or quite widespread, expression patterns, with the
