Chromosomal breakpoints in the MLL gene. (A) Distribution of the 118 chromosomal breakpoints between exons 7 and 13 of the MLL gene. Breakpoint locations are denoted by ●. The top row shows the 17 breakpoints in patients with MLL aberrations other than MLL-AF4; the bottom shows 101 with MLL-AF4. Two distinct breakpoint cluster regions (bcrs) can be distinguished. All sequences have been submitted to the EMBL/Genbank/DDBJ database (accession numbers listed in the Supplemental Materials and Methods). (B) Relative frequency of different MLL-AF4 mRNA transcripts as deduced from the chromosomal breakpoint locations.