(TREH-tih-NOYN) A 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.