A gain-of-function mutation in the JAK2 gene (V617F) is commonly found in patients with myeloproliferative disorders (MPD), including polycythemia vera (PV), essential thrombocythemia (ET), and idiopathic myelofibrosis (IMF). The study identified that 9pLOH (loss of heterozygosity on the short arm of chromosome 9) is associated with the V617F mutation, which is a somatic mutation in hematopoietic cells. All 51 patients with 9pLOH had the V617F mutation, while 66 of 193 patients without 9pLOH were heterozygous for V617F. The frequency of V617F was highest in PV (65%) and lowest in ET (23%). The V617F mutation provides a proliferative and survival advantage to hematopoietic progenitors, leading to clonal expansion. Patients with V617F had a longer disease duration and higher rates of complications such as fibrosis, hemorrhage, and thrombosis, and were more likely to receive cytoreductive therapy. The mutation is likely caused by mitotic recombination, which leads to 9pLOH and homozygosity for V617F. The study suggests that V617F is a dominant gain-of-function mutation contributing to the pathogenesis of MPD. The findings indicate that the V617F mutation is a key driver in the development of MPD and may serve as a basis for new molecular classifications and targeted therapies.A gain-of-function mutation in the JAK2 gene (V617F) is commonly found in patients with myeloproliferative disorders (MPD), including polycythemia vera (PV), essential thrombocythemia (ET), and idiopathic myelofibrosis (IMF). The study identified that 9pLOH (loss of heterozygosity on the short arm of chromosome 9) is associated with the V617F mutation, which is a somatic mutation in hematopoietic cells. All 51 patients with 9pLOH had the V617F mutation, while 66 of 193 patients without 9pLOH were heterozygous for V617F. The frequency of V617F was highest in PV (65%) and lowest in ET (23%). The V617F mutation provides a proliferative and survival advantage to hematopoietic progenitors, leading to clonal expansion. Patients with V617F had a longer disease duration and higher rates of complications such as fibrosis, hemorrhage, and thrombosis, and were more likely to receive cytoreductive therapy. The mutation is likely caused by mitotic recombination, which leads to 9pLOH and homozygosity for V617F. The study suggests that V617F is a dominant gain-of-function mutation contributing to the pathogenesis of MPD. The findings indicate that the V617F mutation is a key driver in the development of MPD and may serve as a basis for new molecular classifications and targeted therapies.