Chronic myeloid leukemia (CML) is a slow-growing cancer of the blood and bone marrow in which the body produces too many white blood cells. It accounts for roughly 2.1 new cases per 100,000 people each year, with a median age at diagnosis of 67. What makes CML unusual among cancers is that nearly every case traces back to a single, well-understood genetic accident, which has made it one of the most treatable forms of leukemia.
What Causes CML
CML begins when two chromosomes inside a bone marrow cell swap pieces of their DNA. Specifically, a section of chromosome 9 breaks off and attaches to chromosome 22, and a piece of chromosome 22 moves to chromosome 9. The shortened chromosome 22 that results is called the Philadelphia chromosome, and it carries a new, fused gene called BCR-ABL1.
This fused gene acts like a switch stuck in the “on” position. It produces a protein that constantly signals bone marrow cells to keep dividing and to resist their normal programmed death. Multiple growth pathways get activated at once, driving the bone marrow to churn out immature and abnormal white blood cells far faster than it should. These cells spill into the bloodstream, crowd out healthy blood cells, and accumulate in organs like the spleen. The translocation isn’t inherited. It happens during a person’s lifetime for reasons that are still not fully understood.
Common Symptoms
Many people with CML have no symptoms at all when they’re first diagnosed. The disease is often caught incidentally on a routine blood test that shows an unusually high white blood cell count. When symptoms do appear in the early (chronic) phase, they tend to be vague: persistent fatigue, malaise, night sweats, and a feeling of fullness or discomfort in the upper left abdomen caused by an enlarged spleen.
As CML progresses, symptoms become more noticeable and more serious. Weight loss, fever, bone pain, abnormal or excessive bleeding, and easy bruising can develop. In the most advanced stage, known as blast crisis, the disease behaves more like an aggressive acute leukemia. Patients may develop severe infections, significant anemia, and in rare cases, neurological symptoms like confusion or visual changes.
The Three Phases of CML
CML is classified into three phases based on the percentage of immature white blood cells (called blasts) in the blood or bone marrow. This staging matters because it determines treatment urgency and outlook.
- Chronic phase: Fewer than 10% blasts in the blood or bone marrow. Most people are diagnosed in this phase, and it’s the most responsive to treatment. Patients can remain in this phase for years with proper therapy.
- Accelerated phase: Between 10% and 20% blasts. The disease is becoming harder to control, and additional chromosome changes may appear in the leukemia cells.
- Blast phase (blast crisis): At least 20% blasts in the blood or bone marrow. The leukemia is now behaving aggressively and requires immediate, intensive treatment.
The goal of modern treatment is to keep CML in the chronic phase indefinitely and prevent it from ever progressing.
How CML Is Diagnosed
Diagnosis typically starts with a complete blood count that reveals a high white blood cell count, often with an unusual mix of immature cells visible on a blood smear. From there, doctors order more specific tests to confirm CML and distinguish it from other blood disorders.
A bone marrow biopsy is a key step. A small sample of bone marrow, usually taken from the back of the hip bone, is examined under a microscope and tested for genetic abnormalities. The definitive diagnosis of CML requires finding either the Philadelphia chromosome or the BCR-ABL1 gene in leukemia cells. Three lab techniques can do this:
- Cytogenetic testing (karyotyping): Examines the chromosomes of bone marrow cells under a microscope to spot the shortened chromosome 22.
- FISH (fluorescent in situ hybridization): Uses fluorescent probes to detect specific pieces of the BCR-ABL1 gene in blood or bone marrow cells.
- PCR (polymerase chain reaction): An extremely sensitive test that detects even tiny amounts of the BCR-ABL1 gene. This is also the primary tool for monitoring how well treatment is working over time.
Treatment With Targeted Therapy
CML treatment was revolutionized in the early 2000s with the introduction of drugs called tyrosine kinase inhibitors, or TKIs. These medications work by blocking the overactive protein produced by the BCR-ABL1 gene, essentially turning off the signal that drives the cancer. Before TKIs, the only curative option was a bone marrow transplant. Today, most people with chronic-phase CML take a daily oral medication and can expect a near-normal lifespan.
The first TKI developed for CML, imatinib, remains widely used and fundamentally changed the disease’s prognosis. However, it is less potent than newer options, and some patients develop resistance to it or don’t respond deeply enough. Second-generation TKIs, including dasatinib, nilotinib, and bosutinib, achieve faster and deeper responses. Dasatinib, for instance, achieved a deep molecular response in 42% of patients at five years compared to 33% with imatinib. Nilotinib showed even stronger results at ten years, with 61% of patients reaching deep molecular response versus 39% on imatinib.
A third-generation TKI, ponatinib, is typically reserved for patients whose disease has resisted other treatments. Newer agents targeting the cancer protein through a different mechanism are also available for patients who don’t respond to or can’t tolerate standard TKIs.
Living With Long-Term Treatment
Because most people with CML take TKIs for years, sometimes indefinitely, side effects are an important consideration. Each TKI has a distinct side-effect profile, and doctors often factor these in when choosing a first treatment.
Imatinib tends to cause milder side effects: fluid retention, nausea, muscle cramps, and rash. Nilotinib is more effective but carries a cardiovascular risk. In one long-term study, 11% of patients on nilotinib experienced cardiovascular events over ten years, compared to 4% on imatinib. Patients on nilotinib also need monitoring for elevated blood sugar and cholesterol. Dasatinib can cause fluid buildup around the lungs (pleural effusion), which affected about 26% of patients cumulatively over five years. A smaller but more serious concern is high blood pressure in the lungs, occurring in roughly 5% of patients by five years. Bosutinib frequently causes digestive issues, with diarrhea affecting 68% of patients in clinical trials.
These side effects don’t mean treatment is worse than the disease. They do mean that you and your doctor will weigh the benefits of deeper responses against the risks specific to your health history. If one TKI causes intolerable side effects, switching to another is a standard approach. Dose adjustments and intermittent treatment schedules can also help maintain effectiveness while improving day-to-day quality of life.
How Treatment Response Is Measured
Once treatment begins, regular blood tests track how much of the BCR-ABL1 gene remains detectable. Responses are measured in “log reductions,” which describe how dramatically the leukemia cells have been reduced.
A major molecular response means that CML cells make up only about 1 in every 1,000 cells (a 3-log reduction, or 0.1%). A deep molecular response goes further, reducing CML cells to roughly 1 in 10,000 or 1 in 32,000 cells. These numbers are reported on an International Scale percentage, and your doctor will track whether that percentage continues to fall over the first months and years of treatment.
Deep molecular response is particularly important because it opens the door to something called treatment-free remission. Some patients who achieve and maintain a deep response for a sustained period can try stopping their TKI under close medical supervision. Not everyone qualifies, and about half of those who try will need to restart treatment. But for those who stay in remission, it represents a genuine possibility of living medication-free.
Outlook and Survival
The prognosis for chronic-phase CML has improved dramatically over the past two decades. Before TKIs, median survival after diagnosis was three to five years. Today, patients diagnosed in the chronic phase who respond well to TKI therapy have life expectancies approaching those of the general population. The critical factor is staying in the chronic phase. Patients who progress to accelerated or blast phase face a much more challenging situation, with fewer treatment options and lower response rates.
Early detection and consistent treatment are what keep the disease manageable. Most people with CML take a pill once or twice a day, get blood tests every few months, and live active, largely unrestricted lives. The shift from a near-fatal diagnosis to a manageable chronic condition makes CML one of the clearest success stories in modern cancer medicine.