From Scientific American:
Researchers believe they may have unlocked the mystery behind a set of blood disorders called myeloproliferative syndromes—precursors to conditions such as leukemia that are triggered by an excess of stem cells. If so, the finding could set the stage for ways to prevent and treat such conditions—some of which can lead to heart disease, abnormal bleeding and even death.
Scientists long believed that these diseases were caused by disruptions in the normal cycle of blood stem cells that prompted them morph into progenitor cells, an intermediate phase when stem cells have been programmed to become a certain type of tissue cell, but have not fully matured into that form. But two new studies published this week in the journal Cell indicate that outside factors rather than flawed cells may be to blame. Specifically, scientists found that blood stem cells may go haywire because of defects in the bone marrow, where they are manufactured.