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There are many different types of leukaemia – some that develop faster (acute) and others that develop more slowly (chronic). This information is about chronic myeloid leukaemia (CML). We have other information about different types of leukaemia.

We also have a booklet on CML, which you can order or download free of charge.

What is CML?

CML is a type of blood cancer that affects your myeloid cells (a type of white blood cell).

Your body needs new blood cells all the time and they are made in the bone marrow (the spongy material inside some of your bones). In a healthy person, ‘starter cells’ called myeloid stem cells develop into myeloid blasts, before eventually turning into mature (fully functioning) blood cells.

If you have CML, your body produces too many myeloid blasts and too many white blood cells called granulocytes. These cells overcrowd the bone marrow, meaning there isn’t enough room for other important blood cells to be made. Some myeloid blasts also enter the bloodstream and, because they haven’t developed properly, stop your body from fighting infection properly.

There is more information about blood cells in our general blood cancer information.

Phases of CML

There are three phases of CML: the chronic phase, the accelerated phase and the blast phase. Most people with CML (nine in 10) are diagnosed in the early (chronic) phase. In this phase your body makes too many granulocytes, but the disease is developing slowly.

If CML is left untreated, it begins to develop more quickly and reaches the accelerated phase. In this phase, you have more myeloid blasts in your blood or bone marrow than in the chronic phase, but this number is still relatively small.

If the leukaemia continues to develop, it will eventually reach the blast phase and transform into an acute form of leukaemia (one that develops more quickly). In this phase, you have too many myeloid blasts in your blood and bone marrow.

Philadelphia chromosome

All cells in your body contain a set of instructions that tell the cell what to do and when to do it. These instructions are stored in structures called chromosomes inside the cells. There are 23 pairs of chromosomes in each cell in your body.

Your chromosomes are made up of a chemical called DNA, which is arranged in sections called genes. Each gene is a code that helps the body make different proteins.

CML is thought to begin when chromosomes nine and 22 get mixed up during cell division (when a cell divides in order to replicate itself). This creates a new, shorter chromosome called the Philadelphia chromosome. During this mix-up, a small part of chromosome nine (containing the ABL1 gene) gets stuck next to a small part of chromosome 22 (containing the BCR gene). In the process, they form a new fusion gene called BCR-ABL1.

The new fusion gene makes a new protein (also called BCR-ABL1). This protein is known as tyrosine kinase, and it causes leukaemia stem cells to divide more often and to live longer than normal blood cells.

We don’t know why this happens, but we do know that you aren’t born with this chromosome and can’t pass it on to your children.
diagram showing the creation of the Philadelphia chromosone caused when chormosones nine and 22 get mixed up during cell division

What causes CML?

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

Although we can’t say what exactly causes CML, there are some things that affect how likely you are to develop the condition.


CML is more common in older people. The median age at diagnosis is 60, which means that half of people with CML are under 60 and the rest are over 60.


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


The only clearly defined risk factor for CML is exposure to a large amount of high-energy (ionizing) radiation. By this, we mean a level similar to what 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.

Does CML run in families?

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

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

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