Publisher Summary Vitamin A (retinol) is an essential component of the diet. It has long been established that retinoids (vitamin A derivatives) are crucial for normal growth, vision, maintenance of numerous tissues, reproduction, and overall survival. This chapter discusses several genetic studies that demonstrate that the retinoic acid receptors (RARs) are essential for the transduction of the retinoid signal, both during development and postnatal life, because their mutation can be lethal. The studies also demonstrate that RARγ is implicated in the specification of the anteroposterior axis of the body, that retinoic acid is effectively used as a signal during morphogenesis, and that RARγ controls the expression of some homeogenes. Some of the developmental teratogenic effects of RA are specifically mediated by RARγ in the lumbosacral region where, in contrast, the RARγ null mutation has no apparent effect on normal development. RARγ null cells with levels of RARαl below a certain threshold may not respond to the RA signal, thus, leading to an altered phenotype, whereas cells with average or higher levels of RARαl would respond to RA, leading to normal development. This is a remarkable illustration of the fact that the level of gene expression in a given cell is not fully determined at any given time.
