Prions--infectious pathogens causing the spongiform encephalopathies.

The novel properties of the scrapie and Creutzfeldt-Jakob disease (CJD) transmissible agents readily distinguish them from viruses and viroids; thus, they have been labeled "prions". The scrapie prion contains a protein(s) which is required for infectivity; recently a 27,000 to 30,000 MW protein which purifies with the prion has been identified. The similarities between the scrapie and CJD agents suggest that CJD is also caused by a prion. Recent studies show that the time courses of both scrapie and CJD are determined by an autosominal dominant gene denoted PID (prion incubation determinant). In congenic mice infected with CJD, PID appears to be located on chromosome 17 in the major histocompatibility complex (H-2) in the D-subregion. Further studies indicate that replication of the scrapie and CJD prions precedes the development of pathological change. These changes share many similarities with those found in a variety of degenerative neurological disorders of unknown etiology.