NF1 and plexiform neurofibroma: Mechanistically, haploinsufficiency of NF1 and subsequent somatic loss of the second NF1 allele in affected cells leads to hyperactive Rat sarcoma - Mitogen activate protein kinase (RAS-MAPK) signaling, promoting abnormal Schwann-cell and glial proliferation that underlies plexiform neurofibroma and OPG formation [7].