The production of blood cells, hematopoiesis, is maintained by a small number of hematopoietic stem cells (HSCs) that have the capacity to differentiate into all hematopoietic lineages, and at the same time self-renew to maintain the HSC pool. Similarly to normal hematopoiesis, leukemias are organized in cellular hierarchies where leukemic stem cells (LSCs) drive tumor growth. LSCs have stem cell characteristics such as self-renewal capacity, quiescence and drug resistance, causing metastasis and relapse. LSCs are therefore a critical priority as targets for therapy. Our objective is to use single-cell methods to dissect the heterogeneity of stem cell populations and discriminate between HSCs and LSCs. This will allow for defining the transcriptional programs that govern HSC fate and explain the molecular basis for leukemogenesis as well as therapy resistance. Ultimately we will use this information to therapeutically target LSCs and thereby improve the treatment of leukemia.