Tretinoin is an important regulator of gene expression during growth and development, and in neoplasms. Tretinoin, also known as retinoic acid and derived from maternal Vitamine A, is essential for normal growth; and Embryonic development. An excess of tretinoin can be teratogenic. It is used in the treatment of psoriases; acne vulgaris; and several other skin diseases. It has also been approved for use in promyelocytic leukemia. Tretinoin is 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.