Dideoxy fingerprinting was used to analyse the 5' flanking region of the neurofibromin (NF1) gene in a panel of 380 neurofibromatosis type 1 (NF1) patients. Five polymorphisms/rare variants were identified at positions -412, - 402, + 16, + 25 and + 132, but control data indicated that these were unlikely to be of pathological significance. Promoter mutations in the NF1 gene are not, therefore, a common cause of NF1. This notwithstanding, a reporter gene assay was performed to determine if these variants could affect the expression of the NF1 gene, and all three changes in the 5'-untranslated region (UTR) (+ 16, + 25, + 132) were found to be associated with a 60-70% increase in reporter gene expression.