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* Context.--Concomitant BCR-ABL1 and JAK2V617F in myeloproliferative neoplasms (MPNs) is rare, and its pathogenesis and clinical significance are unclear. Objective.--To investigate the clonal relationship between the 2 genomic alterations, as well as the clinicopathologic impact. Design.--Retrospective analysis of MPNs with sequential development of BCR-ABL1 and JAK2V617F. Results.--Of 6 cases, 5 had JAK2V617F-positive MPN diagnosed before acquiring BCR-ABL1 years later, and 1 had BCR-ABL1+chronic myeloid leukemia before JAK2V617F-positive myelofibrosis completely replaced the BCR-ABL1+ clone 1 year after tyrosine kinase inhibitor therapy. Among the former group, treatment for the initial MPN involved hydroxyurea, ruxolitinib, and/or supportive care, and the latency to the development of JAK2V617F ranged from 4 to 13 years (median of 9 years). Four cases showed retention of JAK2V617F, whereas BCR-ABL1 emerged as the major clone, including 2 cases that exhibited parallel increases in JAK2V617F and BCR-ABL1 burdens, with both genomic markers exceeding 50%. Three patients received stem cell transplants and demonstrated sustained engraftment, with the genomic markers below detectable levels. Conclusions.--Most MPNs with concomitant JAK2V617F and BCR-ABL1 are actually composite MPNs with a "second hit" residing on a different clone. Rare cases demonstrate a subclone harboring a "double-hit" in a background of a JAK2V617F-positive stem line clone. The probability of a "double-hit" with a BCR-ABL1+ stem line clone is probably reduced by effective tyrosine kinase inhibitor treatment. The treatment often involves combined kinase inhibitors and/or hydroxyurea, but the outcome is unpredictable; hematopoietic stem cell transplantation may be the ultimate therapeutic option for this complicated disease. [ABSTRACT FROM AUTHOR]