Background: Elevation of fatty acid synthase (FAS) in human cancers is often associated with increased tumor aggression. The basic genetic mechanisms leading to increased enzyme content in cancer cells were investigated using cell lines derived from human metastatic breast carcinomas (T47D, Zr75 and SKBr3) and normal human breast epithelium (184A1). Materials and Methods: Western analysis, Northern blotting, [2-C-14]malonyl-CoA incorporation assays, nuclear run-off transcription assays, mRNA decay assays, and poly(A) tail assays were used to measure and compare transcription rates of the FAS gene among the four cell lines. Results: By Western analysis, FAS levels in T47D were 2.6 times lower than ZR75 and SK-Br3, but 6.7 times greater than non-neoplastic 184A1 cells. FAS mRNA levels and specific activity correlated with protein content. In contrast, relative rates of FAS gene transcription were significantly higher in non-neoplastic 184A1 cells than T47D, ZR75 and SKBr3. Stability of message was investigated to explain this discrepancy. The half-life of FAS mRNA in 184A1 cells was 5.6 h, or 4-5-fold less than ZR75 and SKBr3. Poly(A) tail assays showed that FAS mRNA species from 184A1 cells tended to be longer than those of breast cancer cell lines (500-1500 nt versus 500-800 nt, respectively). Conclusion: Breast cancer cell lines contained significantly more FAS enzyme, message and activity than nonneoplastic 184A1 cells. Yet, 184A1 cells exhibited higher rates of FAS gene activation. The discrepancy is explained by faster decay of FAS message in 184A1 cells, suggesting that stabilization of FAS mRNA plays a major role in overexpression of the protein in breast cancer. Increased stability could not be explained simply by degree of polyadenylation. Therefore, some other mechanism (s) must be responsible for conferring stability to FAS RNA species in breast cancer cells.