Molecular markers are helpful in diagnosis, prognosis and management of haematological malignancies. Recently, a single point mutation in the Janus Kinase 2 (JAK2) gene in the Philadelphia-negative myeloproliferative disorders, including polycythemia vera (over 95%), essential thrombocythemia (50%) and primary myelofibrosis (50%) was identified by several groups. This mutation is now considered to have a fundamental role in the pathogenesis of these disorders. A PCR-based test from peripheral blood has become available in India to detect this mutation. Present article discusses the basic aspects of this mutation and its value in diagnosing, prognosticating and treating patients of suspected chronic myeloproliferative disorders.