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 others (RARβ, RARγ and RXRγ) showing complex, tissue-specific expression. Here, I review the expression features of the Rar and Rxr genes during successive steps of early embryonic development, and for later stages in the various differentiating organ systems. These expression features (see Table 1 for a summary) are discussed in relationship with functional data and current models about the roles of retinoic acid signaling during development.
