Supplemental Experimental Procedures Tissue Processing

For histological analysis, we utilized both paraffin sections and frozen sections. For both section techniques, control and mutant littermates at P22, P90, 6-12 months and 12-20 months of age were perfused with 4 % paraformaldehyde (PFA). For paraffin sections, sciatic nerves were dissected, post-fixed in PFA overnight at 4°C, and processed for paraffin embedding. Sciatic nerves were sectioned longitudinally at 5 µm thickness. For frozen sections, sciatic nerves were dissected, post-fixed in PFA for 2 hours at 4°C and transferred to 30 % sucrose overnight at 4°C. Tissue was embedded in using O.C.T. compound (Tissue-Tek) and frozen in crushed dry ice. Sciatic nerves were sectioned longitudinally at 6 or 14 µm thickness. Histology and Tumor Grading Two to three paraffin sectioned slides were stained with hematoxylin/eosin (H&E). Diagnosis for each sciatic nerve lesion was generated based upon histology from H&E stained sections under the light microscopy. The penetrance and frequency of neurofibromas or pre-neoplastic lesions in control and mutant mice was quantified based upon sciatic nerve diagnoses. Quantification of the number of cells per surface area (mm2) in P22 and P90 control and mutant sciatic nerves was performed using ImageJ on digital images of H&E stained sections. For PCR analysis, tissues were dissected in ice-cold PBS and digested with proteinase K as described (Zhu et al., 2005)