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BCR-ABL fusion gene (... FYOO-zhun JEEN)

 A gene formed when pieces of chromosomes 9 and 22 break off and trade places. The ABL gene from chromosome 9 joins to the BCR gene on chromosome 22, to form the BCR-ABL fusion gene. The changed chromosome 22 with the fusion gene on it is called the Philadelphia chromosome. The BCR-ABL fusion gene is found in most patients with chronic myelogenous leukemia (CML), and in some patients with acute lymphoblastic leukemia (ALL) or acute myelogenous leukemia (AML).

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Philadelphia chromosome; three-panel drawing shows a piece of chromosome 9 and a piece of chromosome 22 breaking off and trading places, creating a changed chromosome 22 called the Philadelphia chromosome. In the left panel, the drawing shows a normal chromosome 9 with the abl gene and a normal chromosome 22 with the bcr gene. In the center panel, the drawing shows chromosome 9 breaking apart in the abl gene and chromosome 22 breaking apart below the bcr gene. In the right panel, the drawing shows chromosome 9 with the piece from chromosome 22 attached and chromosome 22 with the piece from chromosome 9 containing part of the abl gene attached. The changed chromosome 22 with bcr-abl gene is called the Philadelphia chromosome.

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