Philadelphia chromosome is a specific chromosomal abnormality that is associated with chronic myelogenous leukemia (CML). It is the result of a reciprocal translocation between chromosome 9 and 22, and is specifically designated t(9;22)(q34;q11). In the formation of the Ph translocation, two fusion genes are generated: BCR-ABL on the Ph chromosome and ABL-BCR on the chromosome 9 participating in the translocation. The BCR-ABL gene encodes a protein with deregulated (uncontrolled) tyrosine kinase activity. The presence of this protein in the CML cells is strong evidence of its pathogenetic role. The Philadelphia chromosome is not passed from parent to child. The presence of this translocation is a highly sensitive test for CML, since 95% of people with CML have this abnormality. However, the presence of the Philadelphia (Ph) chromosome is not sufficiently specific to diagnose CML, since it is also found in acute lymphoblastic leukemia, and occasionally in acute myelogenous leukemia. Fluorescent in situ hybridization (FISH) can be used to look for specific pieces of the bcr-abl gene on chromosomes. This test is rapid; however, the sensitivity of this approach may be limited because in about 5% of normal lymphocytes artifactual colocalisation may be observed. See Philadelphia Chromosome BCR/abl Translocation, by PCR