Myeloid cells of CML are characterized by the Philadelphia chromosome (Ph1) on karyotyping. The Ph1 is a translocation of a portion of the q arm of chromosome 22 to the q arm of chromosome 9, designated t(9:22). This translocation brings the c-ABL proto-oncogene on chromosome 9 in approximation with the BCR (breakpoint cluster) gene on chromosome 22. The hybrid BCR-c-ABL gene encodes a tyrosine kinase that has growth promoting effects through nuclear stimulation.

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