Gerstmann Strausller Scheinker (GSS) human prion disease homogenate is i.c. inoculated into mice exhibiting a proline to leucine alteration at codon 101 of the murine prion protein gene (101LL). This results in a disease with an incubation period of approximately 291 days. Normal brain homogenate i.c. inoculated into 101LL mice which are aged matched are used as controls.