Genes and chromosomes in chronic myeloid leukaemia (CML)

Gene changes happen when something goes wrong when the body is making new blood cells. They are sometimes called gene mutations.

What are genes and chromosomes?

Chronic myeloid leukaemia (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 too much of a substance called tyrosine kinase. Too much tyrosine kinase can cause the bone marrow to make too many white blood cells. It also stops these cells developing into normal white 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. Scientists can see it when they look at the leukaemia cells under a special microscope.

This chromosome is called the Philadelphia (Ph) chromosome. Most people with CML have the Philadelphia chromosome in all the leukaemia cells. It is only in the 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
Image: How the Philadelphia chromosome develops



About our information

  • References

    Below is a sample of the sources used in our chronic myeloid leukaemia (CML). If you would like more information about the sources we use, please contact us at

    Smith G, Apperley J, Milojkovic D, et al. A British Society for Haematology Guideline on the diagnosis and management of chronic myeloid leukaemia. British Journal of Haematology, 2020; 191, 2, Available from [accessed on October 2020].

    Hochhaus A, Saussele S, Rosti G, et al. Chronic Myeloid Leukaemia: ESMO Clinical Practice Guidelines. Annals of Oncology, 2017; 28 (suppl 4), iv41-iv51. Available from [accessed on October 2020].

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    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Senior Medical Editor, Dr Anne Parker, Consultant Haematologist.

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Date reviewed

Reviewed: 01 December 2020
Next review: 01 December 2023

This content is currently being reviewed. New information will be coming soon.

Trusted Information Creator - Patient Information Forum
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Our cancer information meets the PIF TICK quality mark.

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