Fluorescence in situ hybridization (FISH) is a quantitative technique which allows, by means of specific probes, to detect the t(9;22) translocation typical of chronic myeloid leukemia (CML) hematopoietic cells. We have evaluated FISH on interphase nuclei as a method for assessment of residual disease in bone marrow samples from 5 Philadelphia chromosome positive (Ph(+)) CML patients after 12 months therapy with interferon alpha; results were compared with findings obtained by conventional cytogenetics and by polymerase chain reaction (PCR). Our data indicate that FISH is more sensitive than cytogenetics for evaluation of residual disease, being positive in 1 out of 2 cases scored as Ph negative by cytogenetics, but is less sensitive than PCR which turned out to be positive in all patients. As additional advantage over conventional cytogenetics, FISH on interphase nuclei can be performed also on samples lacking metaphases or having poor chromosome spreading or unsatisfactory chromosome banding.

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