Fig 1. Schematic representation of the ABL and the BCR genes disrupted in the t19; 221 (q34; qll). Note the 8604Met gene, which is located immediately upstream of the ABL locus, and whose function is unknown?‘Exons are represented by boxes and introns by connecting horizontal lines. Breakpoints in ABL, illustrated as vertical arrows, almost invariably occur either upstream of exon Ib, between Ib and la, or between la and a2. The BCR gene contains 25 exons, including two putative alternative first (el ‘) and second (e27 exons?‘The breakpoints in BCR usually occur within one of three breakpoint cluster regions (bcr), the location and probable extents of which are shown by the three double-headed horizontal arrows. In exceptional cases the BCR breakpoints fall between m-bcr and M-bcr, within the region indicated by the double-headed dashed-line arrow. The lower half of the figure shows the structure of the various BCR-ABL mRNA transcripts which are formed in accordance with the position of the breakpoint in BCR. Breaks in m-bcr give origin to BCR-ABL mRNA molecules with an ela2 junction. The breaks in M-bcr occur either between exons b2 (e131 and b3 (e141 or between b3 and b4 (e15). generating fusion transcripts with a b2a2 or a b3a2 junction, respectively. Breakpoints in p-bcr, the most 3 ‘cluster region:’ result in BCR-ABL transcripts with an e19a2 junction (originally described as c3a2).