The BCR::ABL1 FISH assay remains a gold standard—highly sensitive, rapid, and extensively validated—critical for the swift detection of the BCR::ABL1 fusion, particularly in diseases where this aberration constitutes the primary pathogenic driver, including chronic myeloid leukemia (CML) and select acute lymphoblastic leukemia (ALL) subtypes. This evidence concerns the gene BCR and acute lymphoblastic leukemia.