Map of the 5′ region of the ABL1 gene. The two alternate first exons of the gene are depicted by a solid box labeled Pa (for exon 1a, transcribed from promoter a) and Pb (for exon 1b, transcribed from promoter b). Arrows indicate the transcription start sites for these two exons. The location of most of the translocation breakpoints in CML is shown on top. For methylation analysis, DNA is restricted with EcoRI (E) and Not I (N) and hybridized using the indicated DNA probe. In this analysis, normal (unmethylated) alleles yield a 5.4-kb fragment, whereas methylated alleles result in a 6.2-kb fragment.