Neurofibromatosis 1 (NF1), NF2, and schwannomatosis are characterized by a predisposition to develop multiple neurofibromas and schwannomas. Currently, there is no blood test to estimate tumor burden in patients with these disorders. We explored whether S100? would act as a biomarker of tumor burden in NF since S100? is a classic immunohistochemical marker of astrocytes, oligodendrocytes and Schwann cells and a small study showed S100? concentrations correlate with the volume of vestibular schwannomas.