Discoveries in cancer research are rarely so exceptional that they both cause immediate changes in the treatment of patients and dramatically advance our understanding of the mechanisms underlying the disease. However, observations of the striking efficacy of all-trans-retinoic acid (tretinoin) in the treatment of patients with acute promyelocytic leukemia, followed closely by the discovery of a specific molecular genetic abnormality in a retinoid receptor, have greatly expanded scientific insight about leukemogenesis and have radically altered treatment strategies in this illness. The observation of tumor cells undergoing maturation as a patient enters remission has also provided a remarkable demonstration of drug-induced . . .