Fatal Familial Insomnia (FFI) is a genetic prion disease caused by a point mutation in the prion protein gene (PRNP) characterized by prominent thalamic atrophy， diffuse astrogliosis and moderate deposition of PrP in the brain. Here， for the first time， we demonstrate that the olfactory mucosa (OM) of patients with FFI contains trace amount of PrP detectable by PMCA and RT-QuIC. Quantitative PMCA analysis estimated a PrP concentration of about 1 × 10 g/ml. In contrast， PrP was not detected in OM samples from healthy controls and patients affected by other neurodegenerative disorders， including Alzheimer's disease， Parkinson's disease and frontotemporal dementia. These results indicate that the detection limit of these assays is in the order of a single PrP oligomer/molecule with a specificity of 100%.