The coincidence of a Janus Kinase 2 (JAK2) V617F positive myeloproliferative neoplasm (MPN) and a BCR-ABL1 positive chronic myeloid leukaemia (CML) is rare. We present a patient whose bone marrow and peripheral blood showed typical features of essential thrombocytemia (ET). However, the normalisation of the white blood cell (WBC) count after therapy with imatinib for a gastrointestinal stromal tumour (GIST) suggests that an underlying CML was masked, as witnessed by the very low levels of BCR-ABL1 at the haematological diagnosis. The question remains if this is a case of two separate myeloid malignancies or a secondary event (BCR-ABL1 fusion) in a primary JAK2 V617F + ET.
(BELG J HEMATOL 2012:3:105–107)