Knowing you have chronic myeloid leukemia (CML) is one thing. You also want to know what causes CML.

Cancer is a disease involving the uncontrolled growth of abnormal cells. Normally, your blood cells are formed, mature, perform their various functions, and then die. Your body is constantly replacing damaged and dying cells so the tissues keep functioning normally. With CML, this process is disrupted.

When your body makes damaged DNA copies

Every cell in your body has the same DNA—the genetic material you inherited from your parents. DNA makes up your chromosomes, and contains genes that control the activity of the cell. Each time a cell divides, its DNA is copied into the new cells. If there is damage to your DNA this is called a mutation. Mutations will be copied to new cells that are formed. This may lead to cancer cells instead of normal cells.

The Philadelphia chromosome: CML's "smoking gun"

CML patients are typically found to have what's known as the Philadelphia chromosome (Ph chromosome), named for the city where researchers first identified it. Every cell has chromosomes—the structures that contain genes—which in turn give instructions to the cells. The Philadelphia chromosome is made when pieces from 2 different chromosomes break off and fuse together, no one knows why, to create a new, abnormal chromosome. This chromosome contains the gene—known as the BCR-ABL gene—that produces the BCR-ABL protein and triggers CML. That's why one of the goals of CML treatment is to reduce the BCR-ABL gene to undetectable levels.

There is no known reason for the genetic changes that cause CML, but there is no evidence that the Philadelphia chromosome is hereditary.

BCR-ABL: The switch that won't turn off

BCR-ABL causes your body to make leukemia cells in your bone marrow that eventually invade your blood. These leukemia cells are immature white blood cells or "blast" cells that behave abnormally as the result of a duplicating error.

Normally, the body is able to control the number of blood cells produced. In the case of CML, BCR-ABL acts like a switch stuck in the "ON" position so it's continuously producing these leukemia cells.

Unlike normal blood cells, leukemia cells don't die when they should. With CML, they build up uncontrollably in the bone marrow and blood. They may crowd out normal white blood cells, red blood cells, and platelets—and the normal bone marrow cells that make them. This obstructs the bone marrow and the normal blood cells.