Recent advances in neuropathology, genotyping, and physiochemical characterization of proteins have allowed for the classification and verification of MM2-thalamic Creutzfeldt-Jakob disease (CJD). CJD is a fatal neurodegenerative illness belonging to the transmissible spongiform encephalopathies, also known as prion diseases. Sporadic CJD is generally classified by the genotype at codon 129 of the prion protein gene and the distinct physiochemical features of the pathologic prion protein (PrP(sc)). The entity is characterized by methionine homozygosity at codon 129, type 2 PrP(sc), and, primarily, thalamic pathology (MM2-thalamic CJD). It shares clinical and pathologic similarities with the genetic prion disorder fatal familial insomnia; the MM2-thalamic phenotype has therefore been called sporadic fatal insomnia (SFI). SFI may also present like other neurodegenerative diseases, and common diagnostic findings that are seen in other forms of sporadic CJD may be absent.