Acute promyelocytic leukemia is a unique subtype of acute myelogenous leukemia characterized by distinct morphologic, cytogenetic, and clinical characteristics. The t(15;17) translocation, which is a hallmark of this disease, results in a transcriptionally active fusion gene-derived from the PML gene from chromosome 15 and the retinoic acid receptor alpha (RARa) gene from chromosome 17. The PML/RARa protein product is responsible for the leukemic phenotype in these patients, but is also able to respond to pharmacologic levels of retinoic acid and induce cell differentiation. Treatment of this leukemia by retinoic acid represents the first example of gene-directed differentiation therapy for acute leukemia and has lead to greater understanding of the pathogenesis of this disease at the molecular level.