Complete cytogenetic and molecular response after imatinib treatment for chronic myeloid leukemia in a patient with atypical karyotype and BCR-ABL b2a3 transcript. 
The molecular hallmark of CML is the BCR-ABL fusion gene, usually with specific breakpoints within ABL intron 1 and BCR introns b2, b3, and e19.  The amplification of the BCR-ABL hybrid gene resulting from additional copies of the Ph chromosome has been identified as a mechanism for imatinib (IM) resistance.  Cytogenetic clonal evolution correlates with the accelerated phase of leukemia, whereas deletions in the derivative chromosome 9 are associated with a poor prognosis.  Relevance in IM therapy is unclear.  We report a case of a 39-year-old male with chronic phase CML.  Cytogenetic studies showed a complex karyotype with additional copies of the Ph chromosome, sextasomy 8, and ASS gene deletion.  An unusual aberrant fusion gene product was derived from the joining of BCR exon 13 (b2) and ABL exon 3 (a3).  During IM treatment, the patient was monitored in 3- to 6-month intervals.  Major cytogenetic response was achieved after 5 months; complete cytogenetic and molecular remission was reached after 8 months; after 22 months, normal karyotype and absence of the BCR-ABL product continued.  Our data seem to confirm the data of others in regards to the b2a3 breakpoint, suggesting a better prognosis, regardless of other unfavorable factors.