Loss of heterozygosity (LOH) and in silico expression analysis were applied to identify genes significantly downregulated in breast cancer within the genomic interval 6q23-25. Systematic comparison of candidate EST sequences with genomic sequences from this interval revealed the genomic structure of a potential target gene on 6q24.3, which we called SAM and SH3 domain containing 1 (SASH1). Loss of the gene-internal marker D6S311, found in 30% of primary breast cancer, was significantly correlated with poor survival and increase in tumor size. Two SASH1 transcripts of approximately 4.4 and 7.5 kb exist and are predominantly transcribed in the human breast, lung, thyroid, spleen, placenta and thymus. In breast cancer cell lines, SASH1 is only expressed at low levels. SASH1 is downregulated in the majority (74%) of breast tumors in comparison with corresponding normal breast epithelial tissues. In addition, SASH1 is also downregulated in tumors of the lung and thyroid. Analysis of the protein domain structure revealed that SASH1 is a member of a recently described family of SH3/SAM adapter molecules and thus suggests a role in signaling pathways. We assume that SASH1 is a new tumor suppressor gene possibly involved in tumorigenesis of breast and other solid cancers. We were unable to find mutations in the coding region of the gene in primary breast cancers showing LOH within the critical region. We therefore hypothesize that other mechanisms as for instance methylation of the promoter region of SASH1 are responsible for the loss of expression of SASH1 in primary and metastatic breast cancer.