We have assembled, annotated, and analyzed a database of over 1700 breakpoints from the most common chromosomal rearrangements in human leukemias and lymphomas. Using this database, we show that although the CpG dinucleotide constitutes only 1% of the human genome, it accounts for 40%-70% of breakpoints at pro-B/pre-B stage translocation regions-specifically, those near the bcl-2, bcl-1, and E2A genes. We do not observe CpG hotspots in rearrangements involving lymphoid-myeloid progenitors, mature B cells, or T cells. The stage specificity, lineage specificity, CpG targeting, and unique breakpoint distributions at these cluster regions may be explained by a lesion-specific double-strand breakage mechanism involving the RAG complex acting at AID-deaminated methyl-CpGs.