Chromosome 3p deletions in breast cancer have been detected at 3p12–p21 by cytogenetic and loss of heterozygosity studies. Recently, we have cloned the FHIT (fragile histidine triad) gene, located at 3p14.2. Abnormalities of the FHIT locus were found in many established cancer cell lines, and the gene was abnormally transcribed in primary tumors of the digestive tract and lung. In this report, we describe the analysis of breast cancer, cell lines, and primary tumors for alterations in transcription of the FHIT gene; about 20% of the samples exhibited altered transcripts. In most of the cases, aberrant transcripts were missing exons. Lack of expression of FHIT mRNA was observed in another 10% of primary tumor samples. These results suggest that alterations in the FHIT gene may play an important role in breast cancer tumorigenesis and suggest that the FHIT gene product functions in the control of the tumorigenic phenotype in a large variety of human neoplasms.