BCR-ABL is a fusion gene formed by the rearrangement of breakpoint cluster region (BCR) on chromosome 22 with the ABL proto-oncogene chromosome 9 leading to the formation of Philadelphia chromosome. This test is used to help diagnose Chronic Myelogenous Leukaemia (CML) and specific types of Acute Lymphoblastic Leukaemia (ALL) by detecting abnormal formation of white blood cells in the bone marrow. It is also used to monitor response to treatment and to detect any resistance to such treatment.