Summary: Accurate monitoring of tumor dynamics and leukemic stem cell (LSC) heterogeneity is important for the development of personalized cancer therapies. In this study, we experimentally induced distinct types of leukemia in mice by enforced expression of Cbx7. Simultaneous cellular barcoding allowed for thorough analysis of leukemias at the clonal level and revealed high and unpredictable tumor complexity. Multiple LSC clones with distinct leukemic properties coexisted. Some of these clones remained dormant but bore leukemic potential, as they progressed to full-blown leukemia after challenge. LSC clones could retain multilineage differentiation capacities, where one clone induced phenotypically distinct leukemias. Beyond a detailed insight into CBX7-driven leukemic biology, our model is of general relevance for the understanding of tumor dynamics and clonal evolution. : In this article, de Haan, Klauke and colleagues prospectively monitor the emergence and kinetics of leukemic stem cell clones in vivo. They used a cellular barcoding approach and uniquely labeled preleukemic stem cells, where leukemias were initiated by Cbx7 overexpression. They document the existence of dormant clones and show that leukemic clones can convert from one lineage to another.