Demonstration of derivative chromosome 9 deletions by a dual-fusion probe system. Metaphase images are shown using a dual-fusion system (dual color, dual fusion probe system; Vysis) in a patient without a derivative chromosome 9 deletion (A) and in patients with a deletion (B-D). The ABL (red) and BCR (green) signals mark the normal 9 and 22 chromosomes, respectively, and the Ph chromosome is marked by a fusion signal. In patients without a deletion, the dual-color/dual-fusion system also labels the derivative chromosome 9 with a fusion signal (A). This fusion signal is missing from the derivative chromosome 9 in patients who carry a deletion of both chromosome 9 and 22 sequences (B). This probe system is also able to detect those patients in whom only chromosome 9 sequences (C) or chromosome 22 sequences (D) are deleted.