Rejoining between 9q+ and Philadelphia chromosomes results in normal-looking chromosomes 9 and 22 in Ph1-negative chronic myelocytic leukemia.

Rearrangement of the breakpoint cluster region (bcr) and the chromosomal location of c-abl and 3'-bcr were studied in two patients with Philadelphia chromosome (Ph1)-negative chronic myelocytic leukemia (CML). One patient (patient 1) had a normal karyotype and the other (patient 2), 46,XY,inv(3)(q21q26). Both patients showed the bcr rearrangement by Southern blot analysis with a 1.2kb 3'-bcr probe. In situ hybridization studies demonstrated the location of the homologous sequences of bcr on chromosome 22 in patient 1, and on chromosomes 9 and 22 in patient 2. These findings indicate that the morphologically normal-looking chromosomes 9 and 22 in patient 2 are the result of a retranslocation between chromosomes 9q+ and 22q-, abnormalities which were first formed by a standard Ph1 translocation.