[The Philadelphia chromosome: clinical and biological significance]

The Philadelphia chromosome (Ph) is an abbreviated chromosome number 22 resulting, in the majority of cases, from a balanced translocation between the 9 and 22 long arms. It is considered a marker of chronic myeloid leukemia and its diagnostic and prognostic value in this disease has been demonstrated. It is also present in a number of patients with acute leukemias of poor prognosis. The Ph arises in a bone marrow progenitor cell and seems to allow the clonal expansion of the malignant cells. The typical 9;22 translocation results in the transposition of the cellular oncogene Abelson in close proximity of chromosome 22 linked critical sequences. This structural change leads to the transcription of an hybrid mRNA coding for an abnormal protein with a tyrosine kinase activity which could play a major role in the leukemogenesis process.