Abnormal neuronal aggregates of α-internexin and the three neurofilament (NF) subunits, NF-L, NF-M, and NF-H have recently been identified as the pathological hallmarks of neuronal intermediate filament (IF) inclusion disease (NIFID), a novel neurological disease of early onset with a variable clinical phenotype including frontotemporal dementia, pyramidal and extrapyramidal signs. α-Internexin, a class IV IF protein, a major component of inclusions in NIFID, has not previously been identified as a component of the pathological protein aggregates of any other neurodegenerative disease. Therefore, to determine the specificity of this protein, α-internexin immunohistochemistry was undertaken on cases of NIFID, non-tau frontotemporal