PRV-1 is a new molecular marker within the Ph-negative chronic myeloproliferative disorders. PRV-1 is a useful, highly sensitive and specific marker in the differentiation between polycythaemia vera (PV) and secondary erythrocytosis (ET), and seems to identify those PV patients presenting in the early phase of the disease with dominating thrombocytosis and thus a clinical phenotype of ET. These PRV-1 positive ET patients can be regarded as having "masked" PV or, more accurately, as having early PV. Moreover, PRV-1 positivity may be associated with a particular risk of thromboembolic complications. The biological role of PRV-1 and the significance of alterations in PRV-1 gene expression levels during treatment remain to be clarified.