BCR and acquired polycythemia vera: The classical BCR::ABL1-negative MPNs, which include essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF), are rare in children, with global incidences for children and young adults estimated to be around 0.6/100,000 patients/year (range 0.004 to 0.9) for ET, 0.18/100,000 patients/year for PV, and 0.53/100,000 patients/year (range 0.003 to 1.5) for PMF [11].