Natural Killer cells (NK) are pivotal cells of innate immunity, that sense defective expression of HLA class I molecules and are complementary to specific cytotoxic T lymphocytes. A defect in NK cell cytotoxicity has been described in some hematopoietic malignancies such as acute myeloid leukemia, multiple myeloma, myelodysplastic syndroms. This defect is at least partially linked to a decreased or absent expression of some activating NK cell molecules, more particularly the so-called Natural Cytotoxicity Receptors (NCRs) NKp30, NKp44 and NKp46. Some old publications have demonstrated defective NK cytotoxicity in myeloproliferative syndroms (chronic myeloid leukemia, primary thrombocytosis, polycythemia vera). The investigators more particularly focused their attention on polycythemia vera (Vaquez's disease), a myeloproliferative disease characterized by the recently describet mutation V617F of the JAK2 tyrosine kinase. The investigators will precise the mechanisms leading to this cytotoxicity defect, the investigators also will evaluate the implication of V617F mutation on NK physiology, and will study the interactions between NK cells and hematopoietic progenitors.