 Neurofibromin is a GTPase-activating protein (GAP) that negatively regulates Ras pathway activity by accelerating hydrolysis of Ras-bound guanosine triphosphate (GTP).Neurofibromin localizes in the cytoplasm; however, some studies have found neurofibromin or fragments of it in the nucleus.Neurofibromin does contain a nuclear localization signal that is encoded by exon 43, but whether or not neurofibromin plays a role in the nucleus is currently unknown.