Based upon molecular allelotyping and comparative genomic hybridization studies, chromosome 15q is the likely location of a tumor suppressor gene important in the pathogeneses of sporadic enteropancreatic endocrine tumors and parathyroid adenomas. Interest has focused on Smad3 as a candidate endocrine tumor suppressor gene because 1) it is localized to 15q and 2) it encodes a TGF beta signaling molecule that has been identified as a binding partner of the multiple endocrine neoplasm type 1 gene product menin, itself involved in enteropancreatic and parathyroid neoplasia. To determine whether Smad3 plays a primary role in development of these tumors, 20 enteropancreatic tumors and 67 parathyroid adenomas were investigated for loss of heterozygosity at DNA markers surrounding Smad3. Twenty percent of enteropancreatic tumors and 24% of parathyroid adenomas showed loss. All 9 coding exons and intron-exon boundaries of the Smad3 gene were then sequenced in genomic DNA from all 20 enteropancreatic and 25 parathyroid tumors, including every case with loss of heterozygosity. No acquired clonal mutations, insertions, or microdeletions in Smad3 were detected in any tumors. Because inactivating somatic mutation is the hallmark of an authentic tumor suppressor, Smad3 is unlikely to function as a classical tumor suppressor gene in the pathogenesis of sporadic parathyroid or enteropancreatic endocrine tumors.