The abnormal eosinophils are part of the leukemic cell population in acute myelomonocytic leukemia with abnormal eosinophils (AML M4Eo) and carry the pericentric inversion 16: a combination of May-Grünwald-Giemsa staining and fluorescence in situ hybridization.

The French-American-British subtype acute myelomonocytic leukemia with abnormal eosinophils (FAB AML M4Eo) with pericentric inversion of chromosome 16 is cytomorphologically defined by a myelomonoblastic blast population and abnormal eosinophils. Until now, it remained an open question whether these abnormal eosinophils are part of the malignant clone or an epiphenomenon. We analyzed five cases of AML M4Eo with inv(16) and combined May-Grünwald-Giemsa staining with fluorescence in situ hybridization using yeast artificial chromosome clone 854E2, which spans the inv(16) breakpoint on 16p. In the case of inv(16), three instead of the normal two hybridization signals can be observed both on metaphase spreads and in interphase cells. With this approach, we were able to show inversion 16 in abnormal eosinophils and, therefore, identified them as a part of the leukemic cell population.