All-trans retinoic acid (ATRA) also known as tretinoin is an active metabolite of vitamin A that has been demonstrated to inhibit the growth of cancer cells, some types of epithelial cells, and vascular smooth muscles. It is medication used for the treatment of acne. Tretinoin capsules are indicated for the induction of remission in patients with acute promyelocytic leukemia (APL), French-American-British (FAB) classification M3 (including the M3 variant), characterized by the presence of the t(15;17) translocation and/or the presence of the PML/RARα gene who are refractory to, or who have relapsed from, anthracycline chemotherapy, or for whom anthracycline based chemotherapy is contraindicated. Tretinoin is for the induction of remission only. For acne it is applied to the skin as a cream or ointment. For leukemia it is taken by mouth for up to three months. The exact mechanism of action of tretinoin in APL and acne treatment is unknown, but is known, that tretinoin activates three members of the retinoid acid (RAR) nuclear receptors (RARα, RARβ, and RARγ) which act to modify gene expression, subsequent protein synthesis, and epithelial cell growth and differentiation. It has not been established whether the clinical effects of tretinoin are mediated through activation of retinoic acid receptors, other mechanisms, or both.