CBFB encodes the beta subunit of the CBFA/CBFB transcription factor complex involved in myeloid differentiation. The chromosomal aberrations inv(16)(p13.1q22.1) and the related translocation t(16;16)(p13.1;q22.1), which have been detected in about 10% of patients with AML (acute myeloblastic leukemia), lead to the fusion of the CBFB gene with the MYH11 (myosin heavy chain 11) gene on 16p13.1. The resulting CBFB-MYH11 fusion gene is involved in a leukemic transformation. The 5' segment of the MYH11 gene is known to be deleted occasionally as a result of the inversion event. AML patients with these genetic rearrangements have a favorable prognosis. Inv(16) may sometimes be difficult to identify using conventional cytogenetic analysis. Accordingly, Fluorescence in situ hybridization proved to be a reliable method overcoming this problem and might consequently be a helpful tool to predict the prognosis of AML patients.