The incidence and significance of Philadelphia negative aberrant clones in patients treated with the second generation of tyrosine kinase inhibitors, including dasatinib, are infrequent. Here we report 2 patients with myelodysplastic syndrome associated with monosomy 7 in Philadelphia negative cells during consecutive imatininb and dasatinb therapy. No previous exposure to chemotherapeutic agents was reported in both patients. In case 1, retrospective assessment of the bone marrow sample collected prior to treatment with dasatinib did not show any abnormal clone, which became evident only after dasatinib was commenced. In case 2, monsomy 7 were found already in 15% Philadelphia negative cells during imatinib treatment and persisted during dasatinib therapy. In the 2 cases described, we assumed that the consecutive imatinib/dasatinib therapy, could abrogate the proliferation of the Philadelphia positive clone and allow pre-existing monosomy 7 clone to emerge.

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