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An important regulator of gene expression, particularly during growth and development and in neoplasms. Retinoic acid derived from maternal vitamin A is essential for normal gene expression during embryonic development and either a deficiency or an excess can be teratogenic. It is also a topical dermatologic agent which is used in the treatment of psoriasis, acne vulgaris, and several other skin diseases. It has also been approved for use in promyelocytic leukemia.