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Chronic myeloid leukemia (CML) is a type of cancer that starts in certain blood-forming cells of the bone marrow (the soft, spongy tissue in the middle of certain bones). It is also known as chronic myelogenous leukemia.

CML happens mostly in adults, but very rarely it happens in children, too. In general, treatment for CML in children is the same as for adults.

How CML starts

CML starts in early forms of white blood cells, which normally help your body fight infections.

In CML, the leukemia cells have an abnormal gene called BCR::ABL1, which causes them to grow out of control. (See What Causes Chronic Myeloid Leukemia.)

The leukemia cells in CML build up in your bone marrow (where new blood cells are made) and then enter your blood. In time, the cells can also settle in other parts of your body, including your spleen.

CML tends to be a fairly slow-growing leukemia, but sometimes it changes into a fast-growing acute leukemia that can be hard to treat.

What is leukemia?

All leukemias, including CML, start in the blood-forming cells of the bone marrow. Normally, these blood-forming cells mature, do their jobs, and then die off. But when one of these cells develops mutations (changes) in its DNA and becomes a leukemia cell, it no longer matures the way it should.

Leukemia cells also don't die when they should. They build up in your bone marrow and crowd out normal cells.

At some point, leukemia cells leave the bone marrow and spill into your bloodstream. This increases the number of white blood cells in your blood. Once they are in your blood, leukemia cells can spread to other organs, where they can keep other cells in your body from working normally.

Knowing the exact type of leukemia you have will help your cancer care team better predict your prognosis (outlook) and plan the best treatment.

What is a chronic leukemia?

Leukemia is either chronic or acute. Chronic leukemia is slower growing. Acute leukemia grows more quickly.

CML is a chronic (slower growing) leukemia.

In a chronic leukemia, the cells mature partly, but they don't mature completely. They are more like normal white blood cells than the cells of acute leukemias.

These cells may look fairly normal, but they're not. They generally don’t fight infection as well as normal white blood cells. The leukemia cells also live longer than normal cells. They build up over time, crowding out normal cells in the bone marrow.

It can take a long time before chronic leukemia causes problems. Most people can live with it for many years. But chronic leukemias tend to be harder to cure than acute leukemias.

What is a myeloid leukemia?

Leukemia is also either lymphocytic or myeloid, depending on which bone marrow cells the cancer starts in.

• Lymphocytic leukemias start in cells that would normally become white blood cells called lymphocytes. These leukemias are also known as lymphoid or lymphoblastic leukemias.
• Myeloid leukemias start in early myeloid cells — the cells that become white blood cells (other than lymphocytes), red blood cells, or platelet-making cells (megakaryocytes). These leukemias are also known as myelocytic, myelogenous, or non-lymphocytic leukemias.

CML is a myeloid leukemia. It starts in early forms of myeloid cells.

To learn more about myeloid cells and how they mature, see Bone Marrow and Blood Cells.