NF1 and plexiform neurofibroma: Our data identifies clinical risk factors for internal nerve sheath tumors in the neurofibromatoses; this information builds upon established genetic factors that influence tumor burden, such as NF1 gene microdeletions (increased tumor burden) [36] and mosaicism for the NF2 gene (decreased tumor burden).[27] In NF1 patients, decreasing number of cafe-au-lait macules correlated with the presence, but not volume, of internal neurofibromas (Figure 3).