Fluorescence in situ hybridization (FISH) technique has been successfully used 
to detect the BCR-ABL gene fusion in chronic myeloid leukemia (CML) with the 
classic form of the Philadelphia chromosome (Ph). We applied FISH to study three 
CML patients showing variant Ph chromosome (either complex or simple type). The 
results demonstrate that the use of a yeast artificial chromosome (YAC)-derived 
probe (D107F9) and a cosmid probe (cos-abl 8), specific for BCR and ABL genes 
respectively, allows also the detection of the BCR-ABL fusion in CML patients 
with variant Ph.