In Philadelphia positive leukemia you have a particular change in the chromosomes of the leukemia cells. Most cells of your body have 23 pairs of chromosomes. Chromosomes are made of DNA. Sections of DNA are called genes. Genes make proteins which have particular jobs to do in the body. For example some genes control how much a cell grows and divides.

When a cell divides to make new cells, the chromosomes normally stay the same. But sometimes mistakes happen.

With Philadelphia positive leukemia, a gene called the ABL1 gene on chromosome 9 breaks off and sticks to a gene called the BCR gene on chromosome 22. It produces a new gene called BCR-ABL1 which causes the cell to make too much of a protein called tyrosine kinase. This protein encourages leukemia cells to grow and multiply.

Doctors treat Philadelphia positive leukemia with a targeted cancer drug such as imatinib, which blocks this protein.