The Philadelphia chromosome was first observed in 1960 and is found in 95% of cases of chronic myeloid leukaemias.
The Philadelphia chromosome is a reciprocal translocation of DNA between the long arms of chromosomes 22 and 9 - t(9;22). The portion of 9q translocated contains abl, a proto-oncogene that is the cellular homolog of a tyrosine kinase coded by the Abelson murine leukaemia virus - ABL.
The abl proto-oncogene is translocated to a specific site on 22q, designated the break point cluster region - BCR. The unregulated tyrosine kinase activity of the bcr-abl fusion protein (p210) disturbs the growth of a clone of haemopoietic stem cells.
Typically the Philadelphia chromosome is not seen in lymphocytes or other cells, but only in the myeloid, megakaryocyte and erythroid cell lines.
The Philadelphia often persiststhroughout remission.
The absence of the Philadelphia chromosome in chronic myeloid leukaemia indicates a poor prognosis.
The Philadelphia chromosome has also been described in about 25% of adults with acute lymphocytic leukaemia (10% of children) and 3% of cases of acute myeloid leukaemia.
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