In mammalian cells, little is known about the nature of recombination-prone regions of the genome. Previously, we reported that the immunoglobulin heavy chain (IgH) mu locus behaved as a hotspot for mitotic, intrachromosomal gene conversion (GC) between repeated mu constant (Cmu) regions in mouse hybridoma cells. To investigate whether elements within the mu gene regulatory region were required for hotspot activity, gene targeting was used to delete a 9.1 kb segment encompassing the mu gene promoter (Pmu), enhancer (Emu) and switch region (Smu) from the locus. In these cell lines, GC between the Cmu repeats was significantly reduced, indicating that this 'recombination-enhancing sequence' (RES) is necessary for GC hotspot activity at the IgH locus. Importantly, the RES fragment stimulated GC when appended to the same Cmu repeats integrated at ectopic genomic sites. We also show that deletion of Emu and flanking matrix attachment regions (MARs) from the RES abolishes GC hotspot activity at the IgH locus. However, no stimulation of ectopic GC was observed with the Emu/MARs fragment alone. Finally, we provide evidence that no correlation exists between the level of transcription and GC promoted by the RES. We suggest a model whereby Emu/MARS enhances mitotic GC at the endogenous IgH mu locus by effecting chromatin modifications in adjacent DNA.