Frequent and extensive deletion during the 9,22 translocation in CML.

Chromosomal translocation is one mechanism by which cellular oncogenes may be activated during tumorigenesis. The translocation of the abl oncogene to the Philadelphia chromosome in chronic myelogenous leukemia (CML) results in a new RNA transcript that fuses sequence from chromosome 22 to sequence from the abl oncogene. This RNA presumably codes for a new abl-related protein product found in CML, the activity of which is different from the normal abl protein. The molecular structure of the translocation varies from patient to patient, and the individual variation in RNA transcript and protein product remains to be defined. This report describes the frequent occurrence of chromosomal deletion within the 9q+ chromosome during these translocations. The location of the deletions suggests that some mechanism maintains the chromosomal breakpoint on the Philadelphia chromosome within a limited region. These deletions complicate the interpretation of Southern blots as a means of detecting the translocation.