While the majority of schwannoma nerve sheath tumors are solitary sporadic tumors, a subset arise as part of heritable tumor predisposition syndromes termed schwannomatosis [[11]]. The identical I SOX10 i mutation was present in each of the four tumors from the first patient, and the identical I SOX10 i mutation was present in both tumors from the second patient (Fig. Based on the observations in these two patients, we conclude somatic mosaicism for I SOX10 i indel mutations causes a form of segmental schwannomatosis lacking other known nerve sheath tumor molecular alterations. The multiple tumors from both patients were found to harbor short in-frame insertion/duplication mutations in the I SOX10 i gene (Supplementary Table S1), similar to those recently discovered in approximately 30% of sporadic solitary schwannomas that were localized at the carboxy-terminal end of the HMG-box DNA binding domain of the encoded homeobox transcription factor (Supplementary Fig. [Extracted from the article]