1. Platelet count ≥450,000 per cubic millimeter; 2. Bone marrow biopsy showing proliferation mainly of the megakaryocytic lineage, with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei. No significant increase or left shift in neutrophil granulopoiesis or erythropoiesis; in rare instances, minor (grade 1) increase in reticulin fibers; 3. Criteria for BCR-ABL1–positive chronic myeloid leukemia, polycythemia vera, primary myelofibrosis, or other myeloid neoplasm not met; 4. JAK2 V617F, CALR, or MPL mutation. Here, JAK2 is linked to myelofibrosis.