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Chronic myelogenous leukemia (CML) is a slow-growing cancer of the bone marrow. It is also sometimes called chronic myeloid, chronic granulocytic, or chronic myelocytic leukemia. In the United States, more than 20,000 people have CML and about 6,000 new cases are diagnosed each year.1

CML mostly affects adults, but a small number of CML patients are children. The cause of CML is unknown.

CML is typically related to the presence of an abnormal chromosome in bone marrow cells called the Philadelphia chromosome. Chromosomes are thread-like strands of DNA that carry genetic information about your body. The Philadelphia chromosome is an abnormal change where DNA from one chromosome exchanges with another. Philadelphia chromosome tells the marrow to make too many white blood cells. Doctors do not know what causes the Philadelphia chromosome to appear.

In rare cases, patients will have all the signs and symptoms of CML, but tests cannot find the Philadelphia chromosome. Doctors think that many of these patients may have one or more other abnormal chromosomes.

There are three phases of CML:

Chronic phase

In the chronic phase, there are more white blood cells in the blood and bone marrow than usual. Most are mature cells that work normally. Most patients are diagnosed in the chronic phase. Many patients with CML in the chronic phase do not notice any symptoms at all, and a doctor may first see a change in a blood test during a routine visit. Without treatment, patients in the chronic phase will get worse, and will eventually enter the accelerated phase.

Accelerated phase

In the accelerated phase of CML, there are too many blasts (immature white blood cells) in the marrow, blood, liver, and spleen. These blasts cannot fight infections the way normal white blood cells do. With advances in drug treatments, this phase may sometimes last more than a year before progressing to the next phase, the blast phase.

Blast phase

In the blast phase the number of blasts in the blood stream grows rapidly. As a result, there are much fewer normal blood cells (white blood cells, red blood cells, and platelets), and any symptoms usually become severe.

Learn more:

1. Howlader N, Noone AM, Krapcho M, Garshell J, Neyman N, Altekruse SF, Kosary CL, Yu M, Ruhl J, Tatalovich Z, Cho H, Mariotto A, Lewis DR, Chen HS, Feuer EJ, Cronin KA (eds). SEER Cancer Statistics Review, 1975-2010, National Cancer Institute. Bethesda, MD, http://seer.cancer.gov/csr/1975_2010/, based on November 2012 SEER data submission, posted to the SEER website, April 2013.