The gene responsible for neurofibromatosis type 1 (NF1) encodes a protein similar to GTPase-activating protein (GAP) for the products of ras protooncogenes. Since NF1 protein possesses GAP-like activity, it is suggested that NF1 protein plays a role in the regulation of ras-mediated signal transduction. Impaired function of NF1 protein could therefore, lead to abnormal growth or differentiation of neural crest-derived cells in NF1 patients. Mutations of NF1 gene are also found in sporadic tumors not associated with NF1, supporting the hypothesis that NF1 gene acts as a tumor suppressor gene.