By PCR all had evidence of BCR-ABL transcripts, but none had evidence of the reciprocal ABL-BCR gene product that is seen in 70% of the Ph-positive CML patients.     ///     We used fluorescence in situ hybridization (FISH) to metaphase chromosomes with BCR and ABL cosmid probes in conjunction with the polymerase chain reaction (PCR) to study the mechanism by which the ABL proto-oncogene is inserted into a morphologically normal chromosome 22 in patients with Ph-negative chronic myeloid leukaemia characterized by the BCR-ABL chimeric gene. 