A reciprocal translocation, t(10;22)(q22;q11), resulting in a masked Ph chromosome was identified in a patient diagnosed with chronic myeloid leukemia (CML). Both homologs of chromosome 9 were of the normal pattern. Two signals for the ABL probe, both of them hybridized to chromosome 9, were demonstrated via fluorescence in situ hybridization (FISH). Furthermore, cohybridization with two differently labeled BCR/ABL translocation DNA probes indicated a BCR/ABL fusion apparently located on 9q34. Molecular studies revealed a rearrangement of the BCR region and expression of a chimeric BCR/ABL mRNA of CML configuration. These findings indicate that the BCR/ABL fusion resulted from an unusual relocation of the BCR gene from its normal position on 22q11 to 9q34 adjacent to the ABL gene.