What Is Chronic Myeloid Leukaemia and How Does It Develop?

Author : Nagaraj Joshi | Published On : 23 Oct 2026

Blood cells play an important role in keeping the body healthy, from carrying oxygen to fighting infections. But what happens when the body starts producing too many abnormal white blood cells? This can occur in chronic myeloid leukaemia (CML), a type of blood cancer that begins in the bone marrow and is linked to a specific genetic change.

Key Takeaways

Chronic myeloid leukaemia is a blood cancer that begins in the bone marrow.
Most cases involve the Philadelphia chromosome and the BCR::ABL1 fusion gene.
CML often develops slowly, and some people have no symptoms when it is first detected.
Blood tests and genetic tests help confirm the diagnosis and monitor the disease.

What Happens in Chronic Myeloid Leukaemia?

CML begins in the bone marrow, where blood cells are produced. A genetic change affects certain blood-forming cells, causing them to multiply more than they should.

These cells develop into abnormal white blood cells that accumulate in the blood and bone marrow. As their numbers increase, they can interfere with the production of healthy blood cells.

How Does Chronic Myeloid Leukaemia Develop?

Most cases of CML involve a genetic abnormality called the Philadelphia chromosome. It leads to the formation of a gene that keeps sending signals for blood cells to grow.

The Philadelphia chromosome

Chromosomes contain the genetic material that tells cells how to function. In CML, a piece of chromosome 9 breaks off and joins chromosome 22. The resulting chromosome 22 is shorter than usual and is called the Philadelphia chromosome.

This rearrangement brings two genes, BCR and ABL1, together. They form an abnormal fusion gene known as BCR::ABL1, which plays a central role in the development of CML.

How does the BCR::ABL1 gene cause CML?

The BCR::ABL1 gene produces an abnormal protein that continually signals cells to grow and survive. This protein is a type of tyrosine kinase, an enzyme involved in regulating cell activity.

Normally, signals that control cell growth switch on and off as needed. The abnormal protein in CML remains active, causing affected blood-forming cells to multiply when they should not. This leads to the gradual accumulation of abnormal white blood cells.

The protein also provides a target for treatment. Certain medicines can block its activity and help bring the production of abnormal cells under control.

Why Does Someone Develop CML?

In most cases, doctors cannot identify exactly what causes the genetic change behind CML. It usually develops during a person's lifetime rather than being inherited from their parents. Having a family member with CML does not usually mean that someone has a high risk of developing it.

Age is a recognised risk factor. CML is more common in older adults, although younger people can develop it too. Exposure to high doses of radiation is another known risk factor. However, most people diagnosed with CML have no clear history of such exposure.

There is no established link between CML and everyday lifestyle choices. Most cases cannot be prevented, and a diagnosis does not mean that a person did anything to cause the condition.

What Are the Different Phases of CML?

Doctors classify CML into phases based on findings such as the number of immature blood cells, blood counts and genetic changes. The phase helps them understand how the disease is progressing and plan treatment.

Chronic phase

Most people are diagnosed in the chronic phase, when CML generally progresses slowly. The bone marrow continues to produce blood cells, but abnormal white blood cells may be present in much higher numbers than normal.

Some people feel well and discover they have CML through a routine blood test. Others experience fatigue, night sweats or unexplained weight loss. An enlarged spleen can also cause discomfort or a feeling of fullness in the upper left part of the abdomen.

Treatment aims to control the abnormal cells and prevent the disease from progressing.

Accelerated phase

In the accelerated phase, the disease shows signs of becoming more aggressive. Immature blood cells may increase, blood counts can become harder to control, and further changes may develop in the leukaemia cells.

CML may also respond less effectively to treatment. Doctors use blood counts, molecular tests and other investigations to assess these changes and decide whether treatment needs to be adjusted.

Blast phase

The blast phase, also called blast crisis, is the most advanced phase of CML. Immature blood cells called blasts accumulate in the blood and bone marrow.

The disease behaves more like an acute leukaemia at this stage and can progress rapidly. Treatment is generally more challenging than during the chronic phase.

Not everyone with CML develops blast phase disease. Regular monitoring helps doctors identify changes in the disease and assess how well treatment is working.

How Is Chronic Myeloid Leukaemia Diagnosed?

CML can be found during a routine blood test, sometimes before a person develops symptoms. If the results suggest the condition, doctors use further tests to confirm the diagnosis.

Complete blood count (CBC): Measures the levels of different blood cells. A high white blood cell count may raise suspicion of CML.
Blood smear: Allows doctors to examine blood cells under a microscope for abnormalities.
Genetic tests: Detect the BCR::ABL1 gene or Philadelphia chromosome, which are key indicators of CML.
Bone marrow examination: May be used to examine blood-forming cells and assess the disease.

A high white blood cell count alone does not confirm CML. Infections and other conditions can also raise the count, so genetic testing is essential for establishing the diagnosis. Blood counts and molecular tests are then repeated to monitor the disease and measure the response to treatment.

How Is CML Treated?

CML is mainly treated with targeted medicines called tyrosine kinase inhibitors (TKIs). These medicines block the abnormal BCR::ABL1 protein that drives uncontrolled leukaemia cell growth.

Imatinib is an established treatment for CML. Imatinib-containing medicines, including Imanib 400 mg Tablet, may be prescribed under specialist supervision. Other tyrosine kinase inhibitors are also available.

Regular blood tests and molecular tests help doctors check whether treatment is working. Patients should continue taking their prescribed medicines and attend follow-up appointments, even when they feel well.

Can People With CML Live a Long Life?

Many people with chronic myeloid leukaemia can live for many years with the condition. Targeted medicines such as imatinib have significantly improved outcomes, allowing many patients to keep CML under control over the long term.

The outlook depends on the CML phase at diagnosis and how well treatment works. Some patients may eventually be eligible to stop treatment under close medical supervision, but this is not suitable for everyone.

Disclaimer: This article is intended for educational purposes only. It does not replace professional medical advice, diagnosis or treatment. Anyone with persistent or unexplained symptoms, or abnormal blood test results, should seek medical evaluation.