Genes and chromosomes in chronic myeloid leukaemia (CML)

CML develops when the body makes an new abnormal gene by accident.

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 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.

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

    European Leukemia Net. Recommendations for the management of chronic myeloid leukemia. 2013.

    Hoffbrand V, and Moss P. Hoffbrand’s essential haematology. 7th edition. 2016.

    National Institute for Health and Care Excellence. Leukaemia (chronic myeloid) – dastatinib, nilotinib and standard dose imatinib for the first-line treatment of chronic myeloid leukaemia (part review of technology appraisal guidance 70). April 2012.

    National Institute for Health and Care Excellence. Technology appraisal guidance. 401/426/425.

    DeVita V, Lawrence T and Rosenberg S. 2016. Lymphomas and leukemias. From Cancer: principles and practice of oncology.


  • Reviewers

    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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