The hallmark of CML is the presence of the BCR::ABL1 fusion gene, created by the t(9;22) which gives rise to the Philadelphia (Ph) chromosome abnormality, and diagnosis depends on its detection or inference, by cytogenetics (conventional chromosome banding analysis (CBA) or fluorescent in situ hybridization (FISH)) and/or by reverse transcriptase polymerase chain reaction (RT-PCR) assays. The gene discussed is BCR; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.