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Loss of constitutive heterozygosity at 11q23 has been detected in various human solid tumors. Here, we described the analysis of a series of normal and tumor pairs from 110 breast carcinomas for the presence of loss of heterozygosity at 11q23 loci. The overall frequency of LOH was 48%, con®rming the importance of deletions at 11q23 in breast tumorigenesis. Previously, we have identi®ed two independent regions of LOH at 11q23, the LOH region 1 at 11q23.1 and the LOH region 2 at 11q23.3. The most telomeric region was recently re®ned between loci D11S1345 and D11S1316, a region of about 1 Mb. However, the LOH region 1, most centromeric, was still not ®nely re®ned: the boundaries were de®ned by loci D11S2000 and D11S897, separated by about 8 Mb. Here, we re®ned its boundaries between loci D11S1347 and D11S927, a region of about 2 Mb. We have mapped 11 expressed sequence tags (ESTs) within this region and excluded another 20. This study represents a further step toward the identi®cation of the putative tumor suppressor gene found within the LOH region 1 at 11q23.1.Keywords: LOH at 11q23; breast cancer Loss of heterozygosity studies are believed to pinpoint the location of cancer recessive genes, and several studies have indicated that the long arm of chromosome 11 harbors genes that are deleted during tumorigenesis. Deletions at 11q23 were detected in a variety of human neoplasms, including breast (Carter et al