Despite the recent success of tyrosine kinase inhibitors (TKIs) in the treatment of chronic myeloid leukemia (CML), approximately 2-17% of patients develop clonal cytogenetic changes in the Philadelphia-negative (Ph(-)) cell population. A fraction of these patients, in particular those displaying trisomy 8 or monosomy 7, are at risk of developing a myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). Consequently, there is a need to characterize the clinical features of such cases and to increase our understanding of the pathogenetic mechanisms underlying the emergence of clonal cytogenetic changes in Ph(-) cells. To date, most cases reported have received treatment with imatinib. Here we describe the case of a patient with CML who developed monosomy 7 in Ph(-) cells during dasatinib therapy. At 20 months after dasatinib initiation, the patient developed MDS, which rapidly progressed into AML. Genome-wide 500K SNP array analysis of the monosomy 7 clone revealed no acquired submicroscopic copy number changes. Given the strong association between monosomy 7 and mutation of genes involved in the RAS pathway in juvenile myelomonocytic leukemia, we also screened for pathogenetic variants in KRAS, NRAS, and PTPN11, but did not detect any changes.