In 2/16 patients, FISH could not exclude an intact ABL1-BCR fusion gene.     ///    "Thus, most CML patients without ABL1-BCR transcript could be characterized cytogenetically to belong to two major subgroups: a silent ABL1-BCR gene was attributed to a deletion in der(9)t(9;22) in 56% of the investigated patients or to variants of a standard t(9;22) (approximately 31%). "    ///    "Conversely, none of the 50 patients with an ABL1-BCR transcript exhibited a variant t(9;22) in GTG-banding analysis. "    ///    " The objective of this study was to characterize the ABL1-BCR fusion gene in 76 BCR-ABL1-positive chronic myeloid leukemia (CML) patients regarding expression as well as genomic status, to assess the frequency of ABL1-BCR gene deletion in these patients, which has been reported to be an adverse prognostic factor in Philadelphia chromosome-positive CML. "    ///    ABL1-BCR gene status was analyzed by FISH in 16 CML patients with no ABL1-BCR transcript.     ///    "Thus, genomic aberrations such as deletions or complex genomic rearrangements are the basic and most frequent cause for ABL1-BCR RNA negativity in CML. "    ///    The heterogeneity of the underlying molecular mechanisms may explain divergent clinical implications described for patients with an ABL1-BCR deletion and those with no ABL1-BCR transcript..     ///    Patients were analyzed for ABL1-BCR 1b-b3 and/or 1b-b4 transcription by RT-PCR analysis.     ///    FISH revealed a partial or total deletion of the ABL1-BCR gene in 9/16 and localized the 5' portion of ABL1 and the 3' portion of BCR at separated loci in 5/16 patients. 