of chromosome 17 which encodes a protein known as neurofibromin (not to be confused with the disorder itself) which plays a role in cell signaling. The Neurofibromin 1 gene is a negative regulator of the Ras oncogene signal transduction pathway. It stimulates the GTPase activity of Ras. It shows greater affinity for RAS p21 protein activator 1, but lower specific activity. The mRNA for this gene is subject to RNA editing (CGA->UGA->Arg1306Term) resulting in premature translation termination. Alternatively spliced transcript variants encoding different isoforms have also been described for this gene. In 1989, through linkage and cross over analyses, neurofibromin was localized to chromosome 17. It was localized to the long arm of chromosome 17 by chance when researchers discovered chromosome exchanges between chromosome 17 with chromosome 1 and 22. This exchange of genetic material presumably caused a mutation in the neurofibromin gene, leading to the NF1 phenotype. Two recurrent microdeletion types with microdeletion breakpoints located in paralogous regions flanking NF1 (proximal NF1-REP-a and distal NF1-REP-c for the 1.4 Mb type-1 microdeletion, and SUZ12 and SUZ12P for the 1.2 Mb type-2 microdeletion), are found in most cases. Structure The neurofibromin gene was soon sequenced and found to be 350,000 base pairs in length. However, the protein is 2818 amino acids long leading to the concept of splice variants. For example, exon 9a, 23a and 48a are expressed in the neurons of the forebrain, muscle tissues and adult neurons respectively. Homology studies have shown that neurofibromin is 30% similar to proteins in the GTPase activating protein (GAP) family. This homologous sequence is in the central portion of neurofibromin and being similar to the GAP family is recognized as a negative regulator of the Ras kinase. Additionally, being such a large protein, more active domains of the protein have been identified. One such domain interacts with the protein adenylyl cyclase, and a second with collapsin response mediator protein. Together, likely with domains yet to be discovered, neurofibromin regulates many of the pathways responsible for overactive cell proliferation, learning impairments, skeletal defects and plays a role in neuronal development. Inheritance and spontaneous mutation The mutant gene is transmitted with an autosomal dominant pattern of inheritance, but up to 50% of NF-1 cases arise due to spontaneous mutation. The incidence of NF-1 is about 1 in 3500 live births. Related medical conditions Mutations in the NF1 gene have been linked to NF-1, juvenile myelomonocytic