The transmission and development of prion disease is proposed to occur by a process of homotypic conversion from normal to abnormal prion protein, whereby the endogenous PrPC interacts with an incoming PrPSc or de novo PrPSc generated by some unknown post-translational process (Telling et al., 1995; Kaneko et al., 1997). The gene discussed is PRNP; the disease is prion disease.