Chronic myeloid leukemia (CML) is classified differently from other cancers because it affects blood-forming tissue in the bone marrow rather than forming a solid tumor. Instead of stages, CML is categorized into phases based on the disease's progression.

• In the chronic phase, fewer than 10% of the blood and bone marrow cells are blast cells. This is the most common phase at diagnosis.
• In the accelerated phase, leukemia cells (abnormal granulocytes with the BCR-ABL gene) increase in number, and 10–19% of the blood and bone marrow cells are blast cells. These abnormal cells crowd out healthy red and white blood cells and platelets, leading to more noticeable symptoms.
• The blast phase of CML, now less common due to effective targeted therapies, involves a rapid increase in blast cells. When CML progresses to this phase, it behaves more aggressively. In this phase, 20% or more of the blood and bone marrow cells are blast cells, significantly worsening the condition.