Leukemia is a cancer of the blood. Leukemia begins when normal blood cells change and grow uncontrollably. Chronic myeloid leukemia (CML) is a cancer of the blood-forming cells, called myeloid cells, found in the bone marrow (the spongy, red tissue in the inner part of large bones). CML most often causes an increase in the number of white blood cells (neutrophils or granulocytes that normally fight infection). It is also sometimes called chronic granulocytic, chronic myelocytic, or chronic myelogenous leukemia. CML makes up about 9% of leukemias.

About the Philadelphia Chromosome

People with CML have an acquired genetic abnormality or mutation in their bone marrow cells, in which part of one chromosome (a long strand of genes) breaks off and reattaches to another chromosome. This is called a translocation. In CML, part of chromosome 9 breaks off and bonds to a section of chromosome 22, resulting in what is called the Philadelphia chromosome or Ph chromosome. The translocation t(9;22) causes two genes called BCR and ABL to become one fusion gene called BCR-ABL. This mutation is found only in the blood-forming cells, not in other organs of the body, and it is not inherited. Therefore, there is no concern about an increased risk to other family members. The BCR-ABL gene causes myeloid cells to make an abnormal enzyme that allows white blood cells to grow out of control.

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 (cells that help the blood to clot) often increase, and the number of red blood cells, which carry oxygen, may decrease.

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Looking for More of an Overview?

If you would like additional introductory information, explore these related items on Cancer.Net:

• ASCO AnswersFact Sheet: Read a one-page fact sheet (available in PDF) that offers an easy-to-print introduction for this type of cancer.

• Cancer.Net Patient Education Video: View a short video led by an ASCO expert in this type of cancer that provides basic information and areas of research.

• Cancer.Net En Español: Read about CML in Spanish. Infórmase sobre leucemia mieloide crónica en español.

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