[Sequential development of BCR::ABL1-positive chronic myeloid leukemia during treatment of JAK2 V617F-positive essential thrombocythemia].

We report a rare case of JAK2 V617F-positive essential thrombocythemia (ET) followed by BCR::ABL1-positive chronic myeloid leukemia (CML) in a 47-year-old woman. Dasatinib induced deep molecular remission; however, persistent thrombocytosis required hydroxyurea. The persistence of the JAK2 V617F mutation despite BCR::ABL1 suppression suggests distinct clonal populations. This case underscores the importance of vigilance for secondary myeloproliferative neoplasms (MPNs) and clonal evolution when a new hematologic phenotype emerges in a patient with an established MPN. The coexistence of JAK2 V617F and BCR::ABL1 mutations is exceedingly rare and presents significant diagnostic and therapeutic challenges.