Overlapping cDNA clones from the translocation breakpoint region (TBR) gene, recently discovered at the neurofibromatosis type 1 locus and found to be interrupted by deletions and at (17; 22) translocation, have been sequenced. A 4 kb sequence of the transcript of the TBR gene has been compared with sequences of genomic DNA, identifying a number of small exons. Identification of splice junctions and a large open reading frame indicates that the gene is oriented with its 5’end toward the centromere, in opposition to the three known active genes in the region. PCR amplification of a subset of the exons, followed by electrophoresis of denatured product on native gels, identified six variant conformers specific to NFl patients, indicating base pair changes in the gene. Sequencing revealed that one mutant allele contains a TC transition changing a leucine to a proline; another NFl allele harbors a CT transition changing an arginine to a stop codon. These results establish the TBR gene as the NFl gene and provide a description of a major segment of the gene.