The reciprocal ABL-BCR gene found in the derivative 9q+ chromosome of the t(9;22) is transcriptionally active in nearly two-thirds of CML patients but has not been shown so far to have a functional role in CML.     ///    "Exceptional CML cases have been described with BCR breakpoints outside the three defined cluster regions, or with unusual breakpoints in ABL resulting in BCR-ABL transcripts with b2a3 or b3a3 junctions, or with aberrant fusion transcripts containing variable lengths of intronic sequence inserts. "