The recent characterization of the alcohol* dehydrogenase Aldh1a2 and the cytochrome P450 Cyp26, two enzymes involved in retinoid metabolism, has helped to explain how bioactive retinoids are made and catabolized. By the elegant definition of an Aldh1a2 null mutation as a dominant suppressor of a Cyp26 null mutation, it is now unequivocally demonstrated that the main function of Cyp26 is to degrade endogenous all-trans retinoic acid rather than to synthesize bioactive hydroxylated retinoids.