FISH systems for detection of the Ph translocation and derivative chromosome 9 deletions. In each case an ideogram and an interphase nucleus are shown for a normal cell, a cell carrying the Ph translocation alone (Ph positive), and a Ph-positive cell carrying a derivative chromosome 9 deletion (Ph positive with der9 deletion). (A) Early probe system. The ABL (red) and BCR (green) loci are labeled with different colored fluorochromes. No difference in the metaphase or interphase patterns is seen between patients who carry a deletion and those who do not. (B) Triple-probe system. In addition to the ABL and BCR loci a third probe containing the ASS gene is labeled in blue. With this system, the translocation again produces a fusion of the BCR and ABL probes on the Ph chromosome but also marks the derivative chromosome 9 with a single blue signal. This signal is missing in patients with deletions. (C) Dual-fusion probe system. The ABL and BCR loci are again labeled with different colored fluorochromes. However in these probe systems the probe size is larger and spans the translocation breakpoints. Both of these probes hybridize to the Ph chromosome and also to the derivative chromosome 9 creating 2 fusion signals in a Ph-positive patient who lacks a deletion but only one fusion signal in a Ph-positive patient with a derivative chromosome 9 deletion. In addition this system can differentiate between loss of chromosome 9 sequences only, chromosome 22 sequences only, or both from the derivative chromosome 9.