Malignant breast carcinoma cell lines are frequently refractory to transforming growth factor beta (TGF-beta)-mediated cell cycle arrest. To identify molecular mechanisms of TGF-beta resistance, we have conducted a comprehensive structural analysis of the TGF-beta receptor types I (TbetaR-I) and II (TbetaR-II) genes in primary human breast carcinomas and associated axillary lymph node metastases. No evidence for loss of expression (n=14) or structural alterations of the TbetaR-II gene (n=30) were identified. However, 2 of 31 primary carcinomas and 5 of 12 lymph node metastases carried a C to A transversion mutation resulting in a serine to tyrosine substitution at codon 387 (S387Y) of the TbetaR-I receptor gene. This TbetaR-I mutant has a diminished ability to mediate TGF-beta-dependent effects on gene expression as compared with wild-type TbetaR-I. S387Y is the first reported mutation in the TbetaR-I gene in human cancer that was primarily associated with lymph node metastases in the present series.