The BCR-ABL fusion gene is formed from a translocation of chromosome 9 and chromosome 22, when sections of chromosome 9 and 22 break off and swap places. The ABL gene from chromosome 9 joins to the BCR gene on chromosome 22, to form the BCR-ABL fusion gene. The altered chromosome 22 with the fusion gene on it is called the Philadelphia chromosome. The BCR-ABL fusion gene/Philadelphia chromosome is found in most patients with chronic myeloid leukemia (CML). It is also found in some patients with acute lymphoblastic leukemia (ALL) or acute myeloid leukemia (AML). The BCR-ABL gene encodes for the BCR-ABL protein.