What is the Philadelphia chromosome?
Named after the city in which it was discovered, the Philadelphia chromosome is a chromosome that develops because of an abnormality in the cells. It is formed when a section of chromosome 9 switches places with a section of chromosome 22, creating an extra-long chromosome 9 and an extra-short chromosome 22. The extra-long chromosome 22 is the Philadelphia chromosome. The Philadelphia chromosome produces a new gene called BCR-ABL (BCR comes from the original chromosome 22 and ABL comes from the original chromosome 9). The BCR-ABL gene instructs the blood cells to produce too much of a certain enzyme called tyrosine kinase. Tyrosine kinase causes blood stem cells (which are different from embryonic stem cells) to produce too many white blood cells. These new white blood cells are not like normal cells in that they grow out of control and do not die as they should. This crowds out healthy blood cells, leading to damage to the bone marrow. Eventually the diseased blood cells infect other
