Targeted hypermutation of immunoglobulin variable region genes occurs in B cells during an immune response, and gives rise to families of related mutant antibodies which are then selected for their binding affinity to the immunizing antigen. Somatic hypermutation predominantly generates point mutations, many of which occur at specific residues (hotspots). The reaction has been linked to transcription and requires the presence of immunoglobulin enhancers^,,, but replacement of the variable gene by heterologous sequences, or the variable region promoter by a heterologous promoter, does not interfere with the mutation process^,. Here we show the existence of abundant DNA double-strand breaks (DSBs) in hypermutating sequences. Generation of the DSBs is coupled to transcription, enhancer-dependent, and correlates with the appearance of nearby mutations. Furthermore, the DSBs are cell-cycle restricted, being found almost exclusively in cells that have completed, or nearly completed, DNA replication. We propose a model for somatic hypermutation in which mutations are introduced into the DNA during repair of DSBs by homologous recombination. The finding of DSBs during somatic hypermutation may help to explain the chromosomal translocations found in some B-cell tumours.