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. The gene discussed is ABL1; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.