Leukemic cells from a patient with Ph-negative chronic myeloid leukemia (CML) had a normal karyotype. M-BCR was rearranged and chromosome in situ hybridization showed an ABL insertion between 5′ and 3′ M-BCR on an apparently normal chromosome 22. The association of 5′ BCR and 3′ ABL at the 5′ junction of the chromosome 9 insert was typical of that found for the BCR-ABL fusion gene in other patients with the standard t(9;22) and CML. With an M-bcr-3′ probe, we cloned and characterized a 3′ junction fragment. Field inversion gel electrophoresis and chromosome in situ hybridization studies using a probe isolated from genomic DNA 5′ of the junction showed that 3′ M-BCR was joined to a region of chromosome 9q34 rich in repetitive sequences and lying some distance 3′ of ABL. The chromosome 9 insert was at least 329 kilobases long and included 3′ ABL and a larger portion of chromosome 9q34. Our results allowed us to exclude transposon- or retroviral-mediated insertion of ABL into chromosome 22. Instead, we favored a two- translocation model in which a second translocation reconstituted a standard t(9;22)(q34;q11) but left the chromosome 9 insert, including 3′ ABL, in chromosome 22.