Definition

An abnormal version of chromosome 22 that contains a fusion of two genes, the ABL gene and the BCR gene. This fusion gene is made when part of the ABL gene on chromosome 9 breaks off and attaches to the BCR gene on chromosome 22. The protein made by the BCR::ABL fusion gene can cause immature white blood cells to grow uncontrollably and build up in the bone marrow and blood. The Philadelphia chromosome is found in the bone marrow cells of almost all people with chronic myelogenous leukemia and some people with acute lymphocytic leukemia or acute myelogenous leukemia.