“…This finding has led to the inclusion of a proteinase K digestion step m all recent methods that have been used in attempts to purify scrapie infectivity (Diringer et al, 1983;Hilmert and Diringer, 1984; Bolton et al, 1982;Prusiner et al, 1983). SAF and infectivity co-purify, at least in part, during the subcellular fractionation of scrapie-infected brain (Diringer et al, 1983;Somerville et al, 1986) and these fibrils, and their major protein component (PrP27 -30) (Diringer et al, 1983;McKinley et al, 1983;Prusiner et al, 1982Prusiner et al, , 1983, have also been described as protease resistant (Diringer et al, 1983;Bolton et al, 1982Bolton et al, , 1984Bolton et al, , 1985Prusiner et al, 1984). The association of PrP27 -30, SAF and infectivity has led to the speculation that SAF are a form of the scrapie agent (Merz et al, 1983a;Diringer et al, 1983;Somerville et al, 1986), and as a scrapie-specific nucleic acid has yet to be found in SAF, that PrP27 -30 is, in itself, the infectious entity (Bolton et al, , 1985McKinley et al, 1983;Prusiner et al, 1982Prusiner et al, , 1983.…”