The gene for neurofibromatosis type 1 (NF1) was identified by positional cloning and found to contain two alternatively spliced exons. The first described alternatively spliced exon (exon 23a) is located within the GAP-related domain of the gene and inserts an additional 63 nucleotides into the NF1 mRNA. The second alternatively spliced exon (exon 48a) is located near the extreme carboxy terminus of the gene and inserts an additional 54 nucleotides into the mRNA. This second isoform, termed 3'ALT, was originally detected while screening a fetal brain cDNA library. Examination of its expression by reverse-transcribed RNA PCR demonstrates high level of expression in cardiac muscle, skeletal muscle and smooth muscle. Trace levels of expression are detected in brain and nerve. The 3'ALT isoform is expressed in fetal cardiac muscle, adult left ventricle and cardiac Purkinje cells. Further confirmation of the existence of this isoform was obtained by blotting the PCR products with a radiolabeled oligonucleotide entirely derived from sequences contained within exon 48a and by direct sequencing of the PCR products. Additionally, this isoform is expressed in muscle tissues from other vertebrate species. The expression of this isoform in muscle suggests that the NF1 gene may play additional tissue-specific roles in muscle development and signal transduction.