Conflicting data suggest that TGF-beta1 can either inhibit or promote the progression of breast cancer. To determine the biological role of TGF beta1 in mammary carcinoma, in this study we examined the gene structure, expression and localization of TGF-beta1 using paraffin-embedded samples from 32 (27 IDC, 1 ILC, 1 DCIS, 1 ADH) breast lesions. Gene mutations in the region coding for the active protein were investigated by PCR-SSCP of exons 5, 6, and 7. mRNA -TGF-beta1 expression and distribution was examined by NISH using cDNA probes generated by RT-PCR and immunohistochemistry. We detected two mutations in exon 6 TGF-beta1 from IDC; and TGF beta1 mRNA and proteins in 28 (87%) of the tumors. Invasive breast carcinomas had more intense TGF-beta1 activity than CIS and than normal tissue adjacent to tumor. TGF beta1 mRNA and proteins were higher at the edge of the tumor than in the center and were also higher in less differentiated breast neoplasms. TGF-beta1 mRNA transcription and protein levels did not correlate either with TGF-beta1 exon 6 mutation or type and grade of differentiation of breast tumors. These observations suggest that TGF beta1 mutations in breast neoplasms might cause loss or inactivation of the growth inhibitory effects of TGF-beta1. They also support the proposed role of TGF-beta1 in the pathogenesis of breast cancer.