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Translocation(15;17) leading to the formation of fusion gene PML/RAR• is the diagnostic hallmark of acute promyelocytic leukemia (APL). Interphase fluorescence in situ hybridization (FISH) is one of the diagnostic tools employed for the detection of PML/RAR• rearrangement. Using a dual color dual fusion (D-FISH) PML/RAR• translocation DNA probe which hybridises both to PML/RAR• and RAR•/PML fusion genes, we characterised the FISH pattern of 52 APL patients at diagnosis and correlated the findings with conventional cytogenetics and RT-PCR analysis. The diagnostic sensitivity of the probe for PML/RAR• was 100%. Seven patients had atypical D-FISH patterns; two had a masked PML/RAR• fusion signal caused by the insertion of PML into RAR• on 17q; 3 had an extra copy of PML/RAR• in the form of isochromosome der(17)(q10)t(15;17) and one had duplication of the normal RAR• gene with an ider(17q) masquerading as i(17)(q10). There was also one case of t(7;17;15) with a typical D-FISH pattern and in which metaphase FISH suggested an unusual 4-point break. In summary, PML/RAR• D-FISH is a highly sensitive method for confirming diagnosis of APL. However D-FISH cannot be solely relied on for the diagnosis of APL owing to atypical patterns which are infrequently observed in cases with additional 17q structural abnormalities, gene insertion and gene duplication.

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