and how the known vertebrate homologues of these genes are expressed normally in differentiation and proliferation pathways as well as abnormal ly in well-defined lymphomagenic and other oncogenic pathways. What emerged from this meeting are a better understanding of the evolution of these gene systems themselves and an elucidation of simpler systems open to more rapid genetic and molecular genetic analysis to reveal the normal functions of these genes and their gene products. Thus we sought new answers to several old questions concerning differenti ation, proliferation, and neoplastic transformation. We gathered together in an unusual format - that of the unique Dahlem Workshops - not just to reiterate data which has recently emerged but to think about how these findings might lead to new approaches for the understanding and therapy of the leukemias and lymphomas. We deliberately chose experts from several different disciplines, ranging from the clinicians who diag nose, describe, and treat these maladies, to the molecular geneticists trying to reduce the analysis of the problem to its simplest variables in the simplest systems possible.