In a number of types of cancer including breast, hepatocellular, and bladder carcinoma, frequent losses of heterozygosity (LOH) on chromosome 11p15 have indicated the presence of one or more tumor suppressor genes in this region. In the present study, we report the detection and characterization of a rearrangement at 11p15 in a sporadic breast carcinoma. Genomic DNA clones encompassing the rearranged region were isolated; localization of both flanking clones to 11p15 by two-color fluorescent in situ hybridization (FISH) indicated that the rearrangement was caused by an interstitial deletion in the affected allele. Although it is uncertain whether the region between the flanking two loci was missing from tumor cells, our result implied that a putative tumor suppressor gene on chromosome 11p15 is located between the loci on either side of the interstitial deletion or may be interrupted by one of the breakpoints.