The repair of double-strand breaks and gaps can be studied in vegetative yeast cells by transforming the DNA with restriction enzyme-cut plasmids. Postulated models for this repair process require the formation of heteroduplex DNA on either side of the region of break or gap repair. We describe the use of restriction site mutations in the his3 gene to detect conversion events flanking but outside of a region of a double-strand break repair. The frequency with which a mutation was converted declined with increasing distance between the mutation and the edge of the gap repair region. The data are consistent with heteroduplex DNA tracts of at least several hundred base pairs adjacent to regions of double-strand break repair.