ABSTRACT
The interest in retinoids from a differentiation and growth point of view catapulted when two independent groups reported in 1987 that all-trans-retinoic acid is a ligand for a nuclear receptor acting as transcription factor (24,25). The receptor was called RAR (retinoic acid receptor) and belongs to the steroid/thyroid hormone receptor superfamily (26). These receptors bind to DNA as dimers. The RAR was later renamed RARa, when two more distinct RARs were described and named RAR~ (27) and RARy (28), respectively. The most potent ligand for these receptors was found to be aH-trans-retinoic acid. A few years ago another retinoic receptor subfamily was isolated (29). Three different members have been described as RXR~. j3, andy, respectively (30). The natural ligand for these receptors was most recently found to be 9-cis-retinoic acid (30-32).
