BRCA1 has been identified as a tumor suppressor gene that is mutated in many cases of inherited breast and ovarian cancer. Recent data suggest that multiple splice forms of BRCA1 exist, but the structure and function of these alternative transcripts have not been elucidated. By sequence analysis of reverse transcription-PCR products, we have determined that a major splice form of BRCA1 expressed in malignant and nonmalignant breast epithelial cells contains an in-frame deletion of 3309 nucleotides from exon 11. A second alternative splice event results in the in-frame deletion of the 123 nucleotides that make up exons 9 and 10. These splice variants are found on polysomes and are therefore predicted to encode 80-85-kDa BRCA1-derived proteins lacking approximately 60% of the internal amino acids that constitute full-length BRCA1.