The neurofibromatosis type 1 (NF1) gene encodes a tumor suppressor protein, neurofibromin, which is expressed at high levels in Schwann cells and other adult tissues. Loss of NF1 expression has been reported in Schwann cell tumors (neurofibrosarcomas) from patients with NF1 and its loss is associated with increased proliferation of these cells. In this report, we describe downregulation of NF1 expression in a single spinal schwannoma from an individual without clinical features of neurofibromatosis type 1 or 2. Barely detectable expression of NF1 RNA was found in this tumor by in situ hybridization using an NF1-specific riboprobe as well as by Northern blot and reverse-transcribed (RT)-PCR analysis. In Schwann cells cultured from this schwannoma, abundant expression of NF1 RNA could be detected by Northern blot and RT-PCR analysis. These results suggest that, in some tumors, expression of NF1 may be downregulated by factors produced within the tumor and may represent a novel mechanism for inactivating these growth suppressing genes and allowing for increased cell proliferation in tumors.