Can you identify the abnormal chromosome in this karyotype? This person has 46 chromosomes with a translocation of material between chromosome 9 and chromosome 22 (commonly known as the Philadelphia chromosome). Detailed studies of the Philadelphia chromosome show that most of chromosome 22 has been translocated onto the long arm of chromosome 9. In addition, the small distal portion of the short arm of chromosome 9 is translocated to chromosome 22. This translocation, which is found only in tumor cells, indicates that a patient has chronic myelogenous leukemia (CML). In CML, the cells that produce blood cells for the body (the hematopoietic cells) grow uncontrollably, leading to cancer.

The connection between this chromosomal abnormality and CML was clarified by studying the genes located on the chromosomes at the sites of the translocation breakpoints. In one of the translocated chromosomes, part of a gene called abl (pronounced A-ble) is moved from its normal location on chromosome 9 to a new location on chromosome 22. This breakage and reattachment leads to an altered abl gene. The protein produced from the mutant abl gene functions improperly, leading to CML.