9-cis retinoic acid signaling: changing partners causes some excitement.

Reports on the role of retinoids in cell metabolism can be traced back almost a century, when the then-unknown vitamin A present in dietary lipids was found to be essential for normal growth {McCollum and Davis 1913; Osborne and Mendel 1913}. Since then, the profound effect of retinoic acid IRA}, a derivative of vitamin A, on development and differentiation has been documented in numerous publications. The effect of retinoids is mediated by nuclear receptors acting as ligand-controlled transcription factors. Although the mechanisms of retinoid-induced trans-activation are many-tiered and might have seemed at times hopelessly complex, recent results have brought some order to a confusing picture.