However, the majority of patients is diagnosed in the setting of a persistent unexplained leukocytosis, and the diagnosis of CP-CML is established by the characteristic Philadelphia (Ph) chromosome abnormality t(9; 22)(q34;q11), assessed either by routine cytogenetics or the detection of a BCR::ABL1 abnormality by fluorescence in situ hybridization or molecular studies. This evidence concerns the gene ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive.