The essential role of PrPC in TSE disease is confirmed by the resistance of PrPnull mice to disease (Bueler et al., 1993); by the reciprocal relationship of PrP gene (PRNP) copy number and incubation period (Bueler et al., 1993, Manson et al., 1994), and by the fact that resistance to sheep scrapie is influenced by polymorphisms of PRNP at codons 136 (V or A), 154 (R or H) and 171 (R or Q) (Goldmann et al., 1994). The gene discussed is PRNP; the disease is scrapie.