A naturally-occurring acid of retinol. Tretinoin binds to and activates retinoic acid receptors (RARs), thereby inducing changes in gene expression that lead to cell differentiation, decreased cell proliferation, and inhibition of tumorigenesis. This agent also inhibits telomerase, resulting in telomere shortening and eventual apoptosis of some tumor cell types. The oral form of tretinoin has teratogenic and embryotoxic properties. Check foractive clinical trials using this agent. ORA nutrient that the body needs in small amounts to function and stay healthy. Tretinoin is made in the body from vitamin A and helps cells to grow and develop, especially in the embryo. A form of tretinoin made in the laboratory is put on the skin to treat conditions such as acne and is taken by mouth to treat acute promyelocytic leukemia (a fast-growing cancer in which there are too many immature blood-forming cells in the blood and bone marrow). Tretinoin is being studied in the prevention and treatment of other types of cancer. Also called all-trans retinoic acid, ATRA, retinoic acid, and vitamin A acid.