(EN) The invention relates to variant V617F of protein tyrosine Kinase JAK2, said variant being responsible for Polycythemia Vera. The invention also relates to a method for the preliminary diagnosis of erythrocytosis and thrombocytosis, thereby enabling same to be related to myeloproliferative syndromes or the detection of variant JAK2 V617F in said myeloproliferative syndromes, such that they can be reclassified in a new nosological group, and to the identification of specific inhibitors and siRNA.