Acute myeloid leukemia (AML) and myeloproliferative neoplasia (MPN) are aggressive diseases of hematopoietic stem and progenitor cells with poor prognosis. Disease progression and relapse in AML are attributed to recently identified (pre)leukemic stem cells (preLSCs). PreLSCs survive induction chemotherapy in AML patients and can thus cause recurrences. Because preLSCs from AML patients are genetically very heterogeneous, they are best isolated from patients with more homogeneous preleukemic conditions such as MPN, which often evolve into AML. Whereas preLSCs have been analyzed on the genome level, the consequences of the genetic mutations for the cellular proteome are unknown at present. Yet, it is the protein expression, which potentially allows targeting of preLSCs in a directed fashion. Therefore, we have collected MPN and AML blood and bone marrow samples, isolated MPN/(pre)leukemic stem cells and developed a new methodology for broad-spectrum proteome analysis of preLSCs. Changes specific to MPN/(pre)leukemic stem cells are tested for correlation with disease progression. Potential target structures are validated functionally.