The position of the Ia CpG island within theBCR-ABL fusion gene is shown. The normal ABL1proto-oncogene (top) comprises 2 alternative promoters (Pb and Pa) each adjacent to exons Ib and Ia, respectively. Ninety percent of CML breakpoints occur in the long intron separating exons Ib and Ia. After the translocation event, the Ia promoter and its associated CpG island become nested within the fusion gene.