The retinoids are a class of chemical compounds that are vitamers of vitamin A or are chemically related to it. Retinoids have found use in medicine where they regulate epithelial cell growth. Retinoids have many important functions throughout the body including roles in vision, regulation of cell proliferation and differentiation, growth of bone tissue, immune function, and activation of tumor suppressor genes. Research is also being done into their ability to treat skin cancers. Currently, alitretinoin (9-cis-retinoic acid) may be used topically to help treat skin lesions from Kaposi's sarcoma, and tretinoin (all-trans- retinoic acid) is used to treat acute promyelocytic leukemia. Types There are four generations of retinoids: First generation include retinol, retinal, tretinoin (retinoic acid), isotretinoin, and alitretinoin Second generation include etretinate and its metabolite acitretin Third generation include adapalene, bexarotene, and tazarotene Fourth generation includes Trifarotene Structure The basic structure of the hydrophobic retinoid molecule consists of a cyclic end group, a polyene side chain and a polar end group. The conjugated system formed by alternating C=C double bonds in the polyene side chain are responsible for the color of retinoids (typically yellow, orange, or red). Hence, many retinoids are chromophores. Alternation of side chains and end groups creates the various classes of retinoids. First and second generation retinoids are able to bind with several retinoid receptors due to the flexibility imparted by their alternating single and double bonds. Third generation retinoids are less flexible than first- and second-generation retinoids and therefore, interact with fewer retinoid receptors. Fourth generation retinoid, Trifarotene, binds selectively to the RAR-y receptor. It was approved for use in the US in 2019. Absorption The major source of retinoids from the diet are plant pigments such as carotenes and retinyl esters derived from animal sources. Retinyl esters are hydrolyzed in the intestinal lumen to yield free retinol and the corresponding fatty acid (i.e. palmitate or stearate). After hydrolysis, retinol is taken up by the enterocytes. Retinyl ester hydrolysis requires the presence of bile salts that serve to solubilize the retinyl esters in mixed micelles and to activate the hydrolyzing enzymes Several enzymes that are present in the intestinal lumen may be involved in the hydrolysis of dietary retinyl esters. Cholesterol esterase is secreted into the intestinal lumen from the pancreas and has been shown, in vitro, to display retinyl ester hydrolase activity. In addition, a retinyl ester hydrolase that is intrinsic to the