Classified as a myeloproliferative disorder (fflp238) with which it shares a number of clinical features. However, it also has certain unique biological properties:

• Characterised in >80% patients by the presence of the Philadelphia chromosome (Ph). Reciprocal translocation between chromosomes 9 and 22, (t9;22)(q34;q11), involving two genes, BCR and ABL that form a fusion gene BCR-ABL on chromosome 22. This produces an aberrant 210 kDa protein that has greater tyrosine kinase activity than the normal ABL protein. This gene is believed to play a role in the pathogenesis of CML but additional genetic changes appear necessary.

• 10% of patients have variant translocations involving chromosome 22 ± 9 and other chromosomes. A further 8% with typical clinical features lack the Ph chromosome, i.e. have Ph-negative CML; half of these have the hybrid BCR-ABL gene: Ph-negative, BCR-ABL-positive CML.

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