In breast cancer, loss of heterozygosity (LOH) has been described on the long arm of chromosome 7 at band q31, suggesting the presence of a tumor suppressor gene in this region. To define the deleted region, we analysed 73 cases of breast cancer and matched normal DNAs with 17 polymorphic markers. A minimal area of LOH was identified as the chromosomal interval flanked by markers D7S687 and metH, spanning a segment of 2 Mb on chromosome 7q31. Of the 73 breast cancer patients studied, all were informative for at least one marker in this region and nine patients showed LOH at one or more loci (12.3%). To define the physical size of the deletion and to ensure the correct interpretation of the LOH deletion studies, we redefined the physical map of markers within this region of 7q31. We present a new physical order for markers at 7q31. More significantly, we have mapped the minimum deletion of 7q31 in the breast cancers studied to date to a physical distance of 1000 kb, contained on a single YAC clone, which includes the MET receptor tyrosine kinase but no other known genes.