Immunoglobulin heavy-chain class-switch recombination (CSR) occurs between highly repetitive switch sequences located upstream of the constant region genes. However, the role of these sequences remains unclear. Mutant mice were generated in which most of the I mu -- C mu intron was deleted, including all the repeats. Late B-cell development was characterized by a severe impairment, but not a complete block, in class switching to all isotypes despite normal germ line transcription. Sequence analysis of the I mu -- C mu intron in in vitro activated-mutant splenocytes did not reveal any significant increase in activation-induced cytidine deaminase (AID)-induced somatic mutations. Analysis of switch junctions showed that, in the absence of any S mu repeat, the Imicro exon was readily used as a substrate for CSR. In contrast to the sequence alterations downstream of the switch junctions, very few, if any, mutations were found upstream of the junction sites. Our data suggest that the core E mu enhancer could be the boundary for CSR-associated somatic mutations. We propose that the core E mu enhancer plays a central role in the temporal dissociation of somatic hypermutation from class switching.