The Philadelphia (Ph) chromosome observed in chronic myeloid leukemia (CML) and acute lymphoblastic leukemia (ALL) is formed by the reciprocal translocation between chromosome 9 and 22, and is specifically designated as t(9;22)(q34;q11). This translocation creates an elongated chromosome 9 and a truncated chromosome 22 known as the Philadelphia chromosome. The translocation results in the juxtaposition of the 5’ end of the BCR gene and the 3’ end of the ABL gene; generating a BCR-ABL chimeric oncogene. Different breakpoints will result in different oncogenic products differing in size; among which the 210kDa product is of upmost importance for diagnosis, therapeutic strategies and monitoring response in CML. Breakpoints in the BCR gene are frequently observed between the 12th - 16th exons. This region is known as the M-BCR (major breakpoint cluster region). The 5 exons within the M-BCR are referred to as b1-b5. The majority of breakpoints in the M-BCR region are in exon 13 (b2) or exon 14 (b3). These breakpoints fuse to the 2nd exon of the ABL gene to form the b3a2 and b2a2 chimeric subtypes. The b3a2 subtype contains the 14th exon and therefore is 75 bp longer than b2a2. Both subtypes encode the 210kDa oncoprotein associated with CML.
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