Leukemia is caused when chromosomes go through translocation, or the transfer of one piece of a chromosome to a non homologous chromosome, and often the two non homologous chromosomes will swap segments. In leukemia, CML is caused by the translocation between one chromosome 9 and one chromosome 22; the translocation designated t(9;22). This results in a chromosome 9 that is longer than normal and a chromosome 22 that is shorter than normal (the Philadelphia chromosome or PH1). The gene that is removed from chromosome 9 is designated c-ABL, and it contains most of the proto-oncogene. A proto-oncogene is gene with one normal cell and one mutated cell; the mutated cell is called an oncogene, and it is dominant. The break in the chromosome 22 occurs in the middle of a gene called BCR. The resulting chromosome, the Philadelphia chromosome, contains a section of the BCR fused with most of the proto-oncogene.
More Information:
The Leukemia and Lymphoma Society