Abstract Chronic myelogenous leukemia (CML) is a stem cell disease which, on a clinical level, progresses from the release from growth control of normally differentiated cells (a preleukemic state) to an acute leukemia. On a molecular level, the evolution of CML to acute leukemia is a multistep process. We propose that an early step, at the stem cell level, is acquisition of the ability for gene movement, which allows subsequent submicroscopic and chromosomal rearrangements that cause changes in the growth characteristics and regulation of the stem cell. A specific platelet DNA polymerase (PDP - reverse transcriptase) may play a role in gene movement. The characteristic reciprocal translocation of chromosomes #9 and #22, causing the activation of the c- abl oncogene, appears to be responsible for the uncontrolled cellular growth. Yet, other growth factors (e.g., platelet derived growth factor) and activated oncogenes (e.g., c-sis) must be responsible for the stimulation, progression, and variability seen during the course of the disease. Because CML is a progressive disease with clinically definable stages, CML appears to be a model system for the study of the molecular basis of the progression of preleukemia to leukemia specifically, and preneoplasia to aggressive neoplasia in general.