Philadelphia positive [Ph +ve] is a term doctors usually use when they talk about a particular form of leukaemia called chronic myeloid leukaemia [CML]. Most people with CML have a genetic abnormality known as a Philadelphia chromosome which can be detected in laboratory tests. (This change in the genes is not hereditary and therefore is not passed on from one generation to the next.)

Every cell in the body has a nucleus that contains chromosomes. Chromosomes are long structures made up of DNA. DNA determines how the cell looks and acts. To make a new cell, the cell needs to make an exact copy of the information inside the nucleus before it divides.

The Philadelphia chromosome develops when part of chromosome 9 (the ABL gene) wrongly attaches to chromosome 22 (the BCR gene) during cell division. This creates a new gene, known as BCR-ABL which produces a specific new protein. The protein causes the production of an enzyme called tyrosine kinase, which leads to the chronic phase of CML by stimulating the production of abnormal blood cells by the bone marrow.

As more is understood about what causes CML doctors hope they will be able to design treatments that target the actual genetic changes causing the disease. An example of this is the new drug Glivec, which works by preventing BCR-ABL working normally.