There are three stages of CML: chronic, accelerated and blast. Most people with CML are diagnosed in the chronic stage, where their body makes too many mature white blood cells (called granulocytes).

All cells in your body contain genes (chromosomes) made up of DNA. If you have CML, something goes wrong with how your chromosomes divide and you’ll have what doctors call a ‘Philadelphia chromosome’, an unusual chromosome found inside all of the cancerous cells.

What is chronic myeloid leukaemia (CML)?

Chronic myeloid leukaemia (CML) is a cancer of the blood. There are three stages of CML and most patients are diagnosed in the early or chronic phase. In this phase your body makes too many mature white blood cells called granulocytes.

If the disease is left untreated it progresses through a period of instability known as the accelerated phase, to the blast phase. In this phase there are too many immature cells (blast cells). You have very few blast cells in your blood or bone marrow if you’re in the chronic phase. The granulocytes can collect in the spleen, making it swell. The spleen is part of your lymphatic system. It’s an organ on your left side, under your ribs.

The blasts can overcrowd the bone marrow, meaning there isn’t enough room for other important blood cells to be made. This can cause many of the signs and symptoms of CML.

What causes chronic myeloid leukaemia (CML)?

You’re not alone. Although it’s quite rare, around 750 people are diagnosed with CML each year in the UK.

When you’re diagnosed with any cancer, one of the first things you might think is: why me? Although the Philadelphia chromosome is present in around 95% of cases of CML, for some people we can’t say what exactly has caused their illness.

However, there are some things which mean you’re at higher risk of developing CML.


CML is more common in older people. The average age at diagnosis is about 60 years. You can get CML at any age, although though it’s very rare in children under 15.


CML is slightly more common in men than women, but we don’t know why.

Family history

There’s no evidence that family members of CML patients are at a higher risk of developing the condition than anyone else.


The only clearly defined risk factor for CML is exposure to high levels of ionizing radiation. When we say ‘high’, it means similar to a level you’d see after an atomic bomb explosion. It’s extremely unlikely that anyone in the UK would be exposed to a level of radiation high enough to increase the risk of getting CML.

Watch Professor Jane Apperley, Consultant Haematologist at Hammersmith Hospital, talk about who gets CML.

The immune system

Your immune system is a network of cells, tissues and organs which protect your body against infection. It is able to react quickly to infections it has seen before: white blood cells and lymphocytes in particular play an important role in this. They circulate around your body in your blood and fight infections.

Most patients with CML don’t get more infections than usual.

The Philadelphia chromosome

You may hear your doctor talk about a ‘Philadelphia chromosome’ or refer to your ‘Philadelphia status’. If you have this chromosome it almost always means you have CML, and it’ll be inside all of your leukaemia cells.

You can’t inherit this chromosome, or pass it on to your children.

All cells in your body contain a set of instructions which tell the cell what to do and when to do it, stored inside the cells in structures called chromosomes. The chromosomes are made up of a chemical known as DNA.

The DNA is arranged in sections called genes. There are 23 pairs of chromosomes in each cell in your body.

When cells divide to form new cells, normally the chromosomes stay the same in each new cell.

CML is thought to begin when an event takes place which doesn’t usually happen. A small part (the ABL1 gene) of chromosome 9 gets stuck next to a small part (the BCR gene) of chromosome 22 by mistake when the cell divides. They form a new fusion gene called BCR-ABL1 and make chromosome 22 shorter than normal. This new, shorter chromosome is called the Philadelphia chromosome, because that’s where it was discovered.

This swapping of genetic material is called a translocation, or chromosomal translocation. This particular translocation is sometimes called t(9;22).

The new BCR-ABL1 gene is important in the development of CML because it makes a new protein (also called BCR-ABL1). This protein is a type of enzyme known as tyrosine kinase, that causes the leukaemia stem cells to divide more often and to live longer than usual.

If doctors can detect the BCR-ABL1 fusion gene, it can help them to diagnose CML – and also monitor your response to treatment. The standard treatment for CML is to use drugs that inhibit the tyrosine kinase enzyme and block its effects.


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