Distribution of breakpoint within the breakpoint cluster region (bcr) in chronic myelogenous leukemia with a complex Philadelphia chromosome translocation.

We analyzed, by Southern blot hybridization, the site of breakpoint within the breakpoint cluster region (bcr) in six patients with a complex Philadelphia chromosome (Ph) translocation and in 23 unselected patients with a standard Ph. The breakpoint was found within the 5.8 kb bcr in all 29 patients. When the bcr was subdivided into four parts, fragments I–IV, based on the restriction enzyme sites, among the six patients with a complex Ph, two had a breakpoint at fragment I, three at fragment II, and one at fragment III. This distribution of breakpoints in patients with a complex Ph did not differ significantly from that in patients with a standard Ph. A deletion of an allele within the bcr was found in three patients (50%) with a complex Ph and in three (13%) with a standard Ph. The internal bcr deletion may be more common in patients with a complex Ph.