NF1 and plexiform neurofibroma: Since patients with the R1809C germline NF1 gene mutation also do not develop plexiform neurofibromas (pNFs)40,44,45 (Fig. 4A), we engineered Nf1+/1809 mice with somatic loss of the conditional Nf1 allele (Nf1flox) in Schwann cell progenitors, the cells of origin of pNFs48,49.