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.
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.
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.