The Philadelphia chromosome is the first cytogenetic abnormality associated with malignant hemopathy, such as chronic myeloid leukemia (CML). This chromosome is a hybrid chromosome produced by a translocation between the BCR gene on chromosome 22 and the ABL gene on chromosome 9, known as translocation t (9; 22). This fusion of the BCR / ABL gene results in the formation of two chimeric proteins called respectively protein 120 (p120) and protein 190 (p190), which significantly increase the activity of the enzyme tyrosine kinase of the ABL gene which is considered the central mechanism in the pathogenesis of CML. Although the BCR / ABL gene is considered a marker of CML, 95% of cases with CML present t (9; 22), it is not exclusive to this entity. There are other hematological disorders associated with this gene such as Acute Lymphoid Leukemia (ALL) (25% of adult cases and 5% in pediatric) and Acute Myeloid Leukemia (1% of cases) and in some isolated myelodysplastic acute syndromes, the presence of the BCR / ABL gene associated with p210 or p190 proteins could be detected.
The detection of these exchanges of chromosomal material can be performed by traditional karyotyping methods; however, there are 5% of cases of CML in which these exchanges remain hidden; In addition, there are cells such as metamyelocytes and neutrophils that are resistant to cell division and consequently to karyotyping. In these cases, the molecular diagnosis is a tool that allows us to detect the alteration when the percentage of the neoplastic cells is low (e.g. Chronic phase), at the same time, it allows the differential diagnosis between CML and myeloid leukemic reaction. It is also useful for bone marrow post-transplant evaluation and therapeutic follow-up.