Platelets are anuclear cells originating from functionally heterogeneous population of haematopoietic stem cells (HSC). The existence of a distinct HSC subtype exclusively supporting long-term platelet production (LT-PP) has been recently demonstrated[1,2]. Decreased platelet counts are common in cancer patients and aged individuals, while direct platelet transfusion has a high risk of adverse reactions. An alternative therapy could rely on donor-independent platelet sources. However, such studies are hampered by lack of quantitative lineage tracing methods in platelets, which (if available) could pinpoint the cell of origin and its molecular characterization. Such a tool could significantly help in development of improved protocols for platelet production from specific subpopulations of cells and testing chemical agents affecting this process. In this context I propose to use a novel, RNA-based cellular barcoding approach to: - Characterize the HSC and progenitors most efficiently producing platelets in vivo through barcode retrieval from RNA in platelets, progenitors, and HSC - To identify key signalling pathways enhancing efficient thrombopoiesis in vivo. - Perform a functional screen of a small molecule library in order to modulate these key pathways in iPSC-derived MKs to maximize ex vivo platelet output. Platelets are small but important blood cells: they stop bleeding at sites of injury, and counter infections by providing a physical barrier to the outside world. Cancer patients frequently have decreased platelet levels due to side effects of chemotherapy, and low platelet levels are also seen in elderly individuals. The result is a potentially life-threatening increase in infections and bleeding problems. Our most effective treatment for this condition is transfusion with donor platelets, but this procedure is risky and some patients respond poorly. My research will improve our understanding of the cells that normally produce platelets, with the goal to increase platelet production. I will use a novel tracking system allowing me to characterize the cells that give rise to platelets. The results from this study will allow us to identify drugs that can efficiently stimulate platelet production, first in cells grown in the lab, and ultimately directly in patients.