a substance closely related chemically to vitamin A. (Vitamin A is retinoic acid and its esters, or retinyl alcohol.) Retinoids regulate growth of epithelial cells (skin, lung, and gut) and are often powerful antioxidants and cancer preventing agents. The early stages of some epithelial cancers can be converted back into normal tissue by some retinoids. Retinoids are also chemically closely related to and made from carotenoids.