The BCR–ABL FISH test is a molecular cytogenetic assay used to detect the BCR–ABL1 gene fusion, resulting from the Philadelphia chromosome t(9;22)(q34;q11) translocation. This fusion gene produces a constitutively active tyrosine kinase that drives uncontrolled proliferation of myeloid cells.

The test employs fluorescently labeled DNA probes that specifically bind to the BCR gene on chromosome 22 and the ABL1 gene on chromosome 9. When the translocation is present, the probes overlap, producing a fusion signal visible under a fluorescence microscope.