What is chronic myeloid leukaemia?

Chronic myeloid leukaemia (CML) is a rare type of cancer. It causes the body to make too many white blood cells.

About 750 people in the UK are diagnosed with CML each year. CML can affect people at any age, but it is more common as people get older. It usually develops very slowly, which is why it is described as a chronic leukaemia. For most people, CML can be well controlled, and they will live a normal life-span.

How CML develops

The information on this page explains how CML develops. This information will help you understand the sections about:

The information is quite technical, so you might need to read it more than once. Don’t worry if it is too much to take in. It is fine to skip it and come back to it another time.

Genes and chromosomes

CML is a cancer of the white blood cells. It develops when some white blood cells start behaving abnormally.

All cells contain a set of instructions that tell them how to behave. These instructions are stored as genes. The genes are organised into structures called chromosomes. Most cells in the body contain 23 pairs of chromosomes.

The BCR-ABL1 gene

New cells are made when a cell divides into two cells. Before a cell divides, it makes a copy of all the instructions stored in the genes on the chromosomes. CML develops when something goes wrong during this copying process. 

A gene called ABL1, which is on chromosome 9, gets stuck to a gene called BCR, which is on chromosome 22. When the ABL1 gene sticks to the BCR gene, it creates a completely new abnormal gene called BCR-ABL1.

This new BCR-ABL1 gene makes a substance called tyrosine kinase. Too much tyrosine kinase causes the bone marrow to make too many white blood cells. It also stops these cells from developing into normal blood cells or dying when they should. These abnormal cells are the leukaemia cells.

The Philadelphia (Ph) chromosome

When the new BCR-ABL1 gene forms on chromosome 22, it changes how the chromosome looks. Doctors can see it when they look at the leukaemia cells under a microscope. 

They call it the Philadelphia chromosome. Most people with CML have the Philadelphia chromosome in all their leukaemia cells.

The Philadelphia chromosome is not inherited. You are not born with it, so you cannot pass it on to your children.

How the Philadelphia chromosome develops
How the Philadelphia chromosome develops

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I’d never heard of CML and didn’t know anything about leukaemia. But my medical team explained what the cancer is, which made it easy to understand.


Back to Understanding chronic myeloid leukaemia (CML)

The blood

To understand chronic myeloid leukaemia, it helps to understand a bit about your blood.

Symptoms of CML

Not everyone has symptoms. Any symptoms that occur are usually mild and similar to common illnesses such as flu.