NF1 and neoplasm: NF1 is a tumor suppressor gene and encodes a 2818-amino acid protein termed neurofibromin that functions as a GTPase-activating protein for the small GTPase RAS (RAS-GAP) and therefore negatively regulates RAS signaling by promoting the conversion of the GTP-bound (active) form of RAS to the GDP-bound (inactive) form.