Polycythemia vera (PV) and essential thrombocythemia (ET) are chronic myeloproliferative neoplasms (MPNs), often associated with mutations in JAK2, CALR, and MPL. Differentiating PV from ET can be challenging in borderline cases, particularly when hemoglobin (Hb), hematocrit (Hct) and erythropoietin (EPO) values are inconclusive. The gene discussed is MPL; the disease is myeloproliferative neoplasm.