The Philadelphia chromosome (Ph) produced by t (9;22) (q34;q11) is of important diagnostic and prognostic significance for hematological tumors. Ph is present in more than 90% of CML patients, 30% of adult ALL patients, 2%-10% of pediatric ALL patients, and a very small number of AML and MM patients. The ABL gene at 9q34 translocates with the BCR gene at 22q11 to form the BCR/ABL and ABL/BCR fusion genes. BCR/ABL fusion gene is an anti-apoptotic gene with high tyrosine kinase activity. It can activate a variety of signal transduction pathways, causing excessive cell proliferation and disturbance of cell regulation. There are two kinds of BCR gene broken sites, almost M-BCR sites in CML, and about 2/3 of them are m-BCR sites in ALL. The prognosis of patients with BCR/ABL fusion gene is poor. In clinical practices, molecular targeted therapeutic drugs can be used selectively according to the presence or absence of BCR/ABL fusion gene.