The abl oncogene is translocated from chromosome 9 to 22 in the creation of the Philadelphia (Ph1) chromosome. This article describes new translocation breakpoints identified in two patients with chronic myelogenous leukemia using Southern blotting and cloned human DNA probes from chromosome 9. The translocation breakpoints on chromosome 9 in both of these patients lie closer to the human cellular abl (c-abl) gene, and the chromosome 22 breakpoints are distributed more widely than previously reported. These data help to define more clearly the chromosomal span of the breakpoints and indicate that some translocations include very little chromosome 9 sequence located 5′ to the c-abl gene.

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