There is no field of medicine in which advances in therapy have been so closely linked to a better understanding of molecular medicine than in the area of hematologic malignancies. For example, recent insights into the understanding of Epstein-Barr virus have led to new treatment options for patients with posttransplant lymphoproliferative disorders, as discussed in the chapter by Dr. Richard Ambinder et al. Similarly, Drs. Slack and Gallagher discuss the explosion of recent information regarding the molecular pathogenesis of acute promyelocytic leukemia. This particular morphologic subtype of acute my eloid leukemia warrants separate discussion because of our increased under standing of the pathogenesis of leukemia, as well as the dramatic advances in outcome that have occurred with differentiation therapy provided by the vitamin .A derivative W-trans retinoic acid, as discussed in the chapter by Drs. Frankel and Powell.