The Philadelphia chromosome is an abnormal chromosome that results from the exchange of portions of genetic material from chromosomes 9 and 22. A mutant gene formed by this exchange codes for a protein whose action causes chronic myeloid leukemia (CML).
Professor Charles Sawyer explains that CML stands for chronic myeloid leukemia, which is a blood cancer and it is different from many cancers because it starts very slowly and patients when they're first diagnosed don't have many symptoms.
BCL-2 is associated with the development of certain forms of leukemia that affect B-cells. If the gene is swapped onto a different chromosome, its protein is over-produced, resulting in B cells that do not function or self-destruct as they should.