Neurofibromatosis type 1 (NF1) is a common autosomal dominant disorder, primarily affecting cells of neural crest origin, and is characterized by café-au-lait skin spots, multiple neurofibromas, and higher incidence of malignancy. A gene linked to NF1 encodes neurofibromin, an established function of which is to stimulate intrinsic GTPase activity of ras protein. The neurofibromin gene gives rise to multiple transcripts generated by alternative splicing, that encode various neurofibromin isoforms. In this study, we have cloned a cDNA encoding a newly identified species of a putative amino-terminal isoform which lacks a large portion of neurofibromin, including the domain related to GTPase-activating protein. This clone carries the insert of 2.7 kb, coding for a protein of 593 amino acid residues, tentatively termed N-isoform 11, whose amino-terminal 574 residues are identical to those of authentic neurofibromin encoded by the eleven exons located at the 5' portion of the gene. Previously, we cloned a cDNA coding for a similar isoform of 551 amino acid residues, termed N-isoform 10, whose amino-terminal portion is encoded by the first ten exons. The molecular weights of these two deduced N-isoforms are consistent with the values determined by in vitro translation of the mRNA transcribed from each N-isoform cDNA. The presence of the amino-terminal isoform(s) suggests the physiological significance of the amino-terminal portion of neurofibromin.