Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder with an incidence of between 1: 3000 and 1: 4000. Common clinical signs include more than six café-au-lait spots, multiple cutaneous neurofibromas and iris Lisch nodules. Rarer are skeletal anomalies, learning disabilities and an increased risk of malignancy. The NF1 gene contains at least 60 exons with intron sizes ranging from 60 bp to more than 40 kb. Despite using different techniques including PTT, SSCP heteroduplex analyses and direct sequencing, only a relatively small number of mutations have been reported world-wide. Using the more sensitive technique of temperature gradient gel electrophoresis (TGGE), we analysed a part of the NF1-GAP-region, namely exon 25, in DNA samples from 131 unrelated patients. We have identified a novel mutation L1425P in exon 25 of the NF1 gene in a 12-year-old boy (clinically diagnosed with NF1 at the age of 7). In contrast to those cases diagnosed with having both GAP-region mutations and malignant tumours, neither the proband nor four clinically affected family members with this mutation showed any evidence of malignancies.