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Approved by the Cancer.Net Editorial Board, 03/2018

About leukemia

Leukemia is a cancer of the blood. Leukemia begins when healthy blood cells change and grow out of control. Chronic myeloid leukemia (CML) is a cancer of the blood-forming cells, called myeloid cells, found in the bone marrow. Bone marrow is the spongy, red tissue in the inner part of large bones. CML most often causes an increase in the number of white blood cells, such as neutrophils or granulocytes, that normally fight infection. It is also sometimes called chronic granulocytic, chronic myelocytic, or chronic myelogenous leukemia.

About the Philadelphia chromosome

People with CML have a genetic mutation or change in their bone marrow cells. It is called a translocation. A translocation is when part of a long strand of genes called a chromosome breaks off and reattaches to another chromosome. In CML, part of chromosome 9 breaks off and bonds to a section of chromosome 22, forming the Philadelphia chromosome or Ph chromosome. The Ph chromosome is made up of 2 genes called BCR and ABL that join together into a single fusion gene called BCR-ABL. It is found only in the blood-forming cells, not in other organs of the body. The BCR-ABL gene causes myeloid cells to make an abnormally activated enzyme. Specifically, it is a tyrosine kinase enzyme. This abnormally activated enzyme is called a fusion protein and allows white blood cells to grow out of control.

This genetic change develops from damage that occurs by chance after a person is born. There is no risk that a person will pass on this gene to their children.

About CML

Ordinarily, the number of white blood cells is tightly controlled by the body—more white blood cells are produced during infections or times of stress, but then the numbers return to normal when the infection is cured. In CML, the abnormal BCR-ABL enzyme is like a switch that is stuck in the “on” position—it keeps stimulating the white blood cells to grow and multiply. In addition to increased white blood cells, the number of blood platelets that help the blood to clot often increase. And, the number of red blood cells, which carry oxygen, may decrease.