NF1 and neurofibromatosis type 1: Neurofibromatosis type 1 is due to a mutation in the tumor-suppressor gene NF1 located on the long (q) arm of chromosome 17, at band 11.2 (17q11.2), resulting in loss of production or reduced function of neurofibromin, a RAS GTPase-activating protein (RAS-GAP) that normally negatively regulates Ras activation.