Recent advances have highlighted extensive phenotypic and functional similarities between normal stem cells and cancer stem cells. This raises the question of whether disease therapies can be developed that eliminate cancer stem cells without eliminating normal stem cells. Here we address this issue by conditionally deleting the Pten tumour suppressor gene in adult haematopoietic cells. This led to myeloproliferative disease within days and transplantable leukaemias within weeks. Pten deletion also promoted haematopoietic stem cell (HSC) proliferation. However, this led to HSC depletion via a cell-autonomous mechanism, preventing these cells from stably reconstituting irradiated mice. In contrast to leukaemia-initiating cells, HSCs were therefore unable to maintain themselves without Pten. These effects were mostly mediated by mTOR as they were inhibited by rapamycin. Rapamycin not only depleted leukaemia-initiating cells but also restored normal HSC function. Mechanistic differences between normal stem cells and cancer stem cells can thus be targeted to deplete cancer stem cells without damaging normal stem cells. Stem cells that initiate and maintain cancers are so like normal stem cells that it's hard to design drugs to target them specifically. This is a serious problem as, for example, damaging blood stem cells in leukaemia therapy can cause haematopoietic failure and death. Now a study of the tumour suppressor PTEN, often inactivated in leukaemia and other cancers, pinpoints a major difference between self-renewal in normal and cancer stem cells. PTEN normally inhibits the phosphatidylinositol-3-OH kinase signalling pathway, limiting cell proliferation and survival. In the absence of PTEN, leukaemic stem cells proliferate, but normal stem cells are depleted. This suggests that PTEN-mimicking drugs may act against leukaemia yet preserve blood stem cells. Indeed, in Pten-deficient mice rapamycin kills leukaemic stem cells but rescues normal stem cell function. A separate study confirms PTEN's role in blood stem cell regulation. The tumour suppressor PTEN is required for maintenance of haematopoietic stem cells, but not leukaemia-initiating cells — this specific effect is mediated by the mTOR pathway and mitigated by rapamycin.